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1 <?xml version="1.0"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 4 5 <html xmlns="http://www.w3.org/1999/xhtml"> 6 <head> 7 <meta name="MSSmartTagsPreventParsing" content="TRUE" /> 8  9  10 11 <title>The Courier Mail Server</title> 12 <link rel="icon" href="icon.gif" type="image/gif" /> 13 </head> 14 15 <body> 16 <a href="#features">Features</a> 17 <a href="install.html">Install</a> 18 19 The Courier mail transfer agent (MTA) is an 20 integrated mail/groupware server based on open commodity 21 protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. 22 Courier provides ESMTP, IMAP, POP3, webmail, and mailing 23 list services within a single, consistent, framework. Individual 24 components can be enabled or disabled at will. The 25 Courier mail server now implements basic web-based 26 calendaring and scheduling services integrated in the webmail 27 module. Advanced groupware calendaring services will follow 28 soon. 29 30 The Courier mail server's source code should compile 31 on most POSIX-based operating systems based on Linux, and 32 BSD-derived kernels. The Courier mail server should also 33 compile on Solaris and AIX, with some help from Sun's or IBM's 34 freeware add-on tools for their respective operating systems. 35 36 The Courier mail server evolved out of several 37 related projects, that merged together (more on that later). The 38 Courier mail server implements SMTP extensions for 39 mailing list management and spam filtering. The Courier 40 mail server can function as an intermediate mail relay, relaying 41 mail between an internal LAN and the Internet, or perform final 42 delivery to mailboxes. The Courier mail server uses 43 <a target="_blank" href= 44 "http://www.courier-mta.org/mbox-vs-maildir/" shape= 45 "rect">maildirs</a> as its native mail storage format, but it can 46 also deliver mail to legacy mailbox files as well. The 47 Courier mail server's configuration is set by plain text 48 files and Perl scripts. Most of The Courier mail 49 server's configuration can now be adjusted from a web browser, 50 using The Courier mail server's web-based administration 51 module. 52 53 The Courier mail server can provide mail services for 54 regular operating system accounts. The Courier mail 55 server can also provide mail services for virtual mail accounts, 56 managed by an LDAP, MySQL, or PostgreSQL-based authentication 57 database. 58 59 Certain portions of the Courier mail server - 60 <a target="_top" href="http://www.courier-mta.org/maildrop/" 61 shape="rect">the mail filtering engine</a>, the <a href= 62 "http://www.courier-mta.org/sqwebmail/" target="_top" shape= 63 "rect">webmail server</a> and the <a target="_top" href= 64 "http://www.courier-mta.org/imap/" shape="rect">IMAP server</a> - 65 are also available as separate, smaller, packages that can be 66 used with other mail servers. 67 68 <h2><a name="features" id="features"></a>Features</h2> 69 70 <ul> 71 <li>Can be configured to function as an intermediate mail 72 relay, or as a mail server that receives mail for multiple 73 domains and makes it accessible to mail clients, or anything in 74 between.</li> 75 76 <li>On servers with multiple IP addresses, optionally assign a 77 <a href="courier.html#multihomed">vanity configuration to 78 Courier for each IP address</a>, making each IP 79 address look like a separate, dedicated, mail server instance, 80 for both incoming and outgoing mail. An alternative limited 81 vanity configuration for outgoing mail only, based on the 82 sending mail client's authenticated login, is possible when 83 multiple IP addresses are not available.</li> 84 85 <li>Web-based administration and configuration tool.</li> 86 87 <li>Local mailboxes can be accessed via POP3. The 88 Courier mail server includes an integrated POP3 89 server.</li> 90 91 <li>Local mailboxes can be accessed via IMAP. The 92 Courier mail server includes an integrated IMAP 93 server.</li> 94 95 <li>A built-in IMAP/POP3 aggregator proxy. It is possible to 96 distribute all mailboxes between multiple servers. A separate 97 server (or a pool of servers) accepts connections from IMAP or 98 POP3 clients, then connects to the right server based on the 99 mailbox the connecting client is logging into.</li> 100 101 <li>Local mailboxes can be accessed via HTTP. The 102 Courier mail server includes an <a href= 103 "/sqwebmail/screenshots.html">integrated webmail 104 server</a>.</li> 105 106 <li>The webmail server includes a personal <a target="_blank" 107 href= 108 "http://www.courier-mta.org/sqwebmail/images/monthly.png">event 109 calendar</a>.</li> 110 111 <li>Uses an efficient <a target="_blank" href= 112 "http://www.courier-mta.org/mbox-vs-maildir/">maildir</a> 113 format as its native mail storage format. Some support is 114 provided for legacy mbox mailboxes.</li> 115 116 <li>Flexible "Sender Policy Framework" support; the ESMTP 117 <code>HELO</code>, <code>MAIL FROM</code>, and the 118 <code>From:</code> header can be validated using SPF.</li> 119 120 <li>DSN, PIPELINING, and 8BITMIME ESMTP extensions. The 121 Courier mail server automatically converts 8-bit 122 messages to 7-bit encoding, for relaying mail to external mail 123 gateways.</li> 124 125 <li>STARTTLS ESMTP extension (as well as 126 IMAP/POP3/ESMTP/Webmail over SSL) in both the client and the 127 server (requires OpenSSL). The ESMTP client can optionally 128 require that the remote server's X.509 certificate is signed by 129 a trusted root CA (a default set of root CAs is provided).</li> 130 131 <li>Experimental TLS/SSL enhancements which are designed to 132 implement a secure mail delivery channel between trusted 133 domains, over an untrusted network. This is implemented by 134 requiring mail to select domains use TLS/SSL connections which 135 require the remote server to present an X.509 certificate 136 signed by a private (not a public) certificate authority. This 137 is pretty much the highest level of security that can be 138 achieved with today's technologies. This doesn't even require 139 DNSsec. Even if the DNS cache is poisoned with MX records that 140 divert mail to a rogue relay, the attacker will not have an 141 X.509 certificate signed by a private CA (this assumes, of 142 course, that the security of the private CA hasn't been 143 breached). This work is mostly complete, but still needs a 144 little testing.</li> 145 146 <li>Message submission protocol (RFC 2476).</li> 147 148 <li>IPv6 support (experimental).</li> 149 150 <li>NOTE: the integrated servers work with maildir-based 151 mailboxes only. There are many existing POP3, IMAP, and webmail 152 servers that provide excellent support for mbox-based 153 mailboxes, so there's no reason to reinvent the wheel. Some 154 popular mbox servers are: <a target="_blank" href= 155 "http://www.eudora.com/qpopper/">Qpopper</a>, <a target= 156 "_blank" href="http://www.washington.edu/imap/">UW-IMAP</a>, 157 and <a target="_blank" href= 158 "http://neomail.sourceforge.net/">NeoMail</a>.</li> 159 160 <li>A faxmail gateway (experimental) that forwards E-mail 161 messages via fax (requires a compatible class 2 faxmodem). The 162 Courier mail server doesn't implement the actual 163 faxing all by itself, actually. The Courier mail 164 server uses additional software (which must be separately 165 installed), to take care of the low-level details. The popular 166 <a target="_blank" href= 167 "http://alpha.greenie.net/mgetty/">mgetty+sendfax</a> package 168 talks to the faxmodem and handles the actual faxing. Conversion 169 of E-mail messages to fax pages is done by <a target="_blank" 170 href="http://www.ghostscript.com/">ghostscript</a>, troff or 171 <a target="_blank" href= 172 "http://www.gnu.org/software/groff/groff.html">groff</a>, and 173 the <a target="_blank" href= 174 "http://netpbm.sourceforge.net/">NetPBM</a> library. The 175 Courier mail server glues all of these pieces together 176 in a seamless manner any time an E-mail message addressed to 177 <code><phonenumber@fax></code> is received. The main 178 textual body of the E-mail message is placed on a cover page, 179 and any attachments are converted to fax image format and 180 transmitted after the cover page. At this time, The 181 Courier mail server knows how to send plain text, PDF, 182 and Postscript attachments. GIF, JPEG, and PNG images can be 183 sent to (one image per page). The additional software packages 184 that were mentioned previously are usually already included in 185 most Linux and BSD installations. In most cases no additional 186 software really needs to be installed in order to get 187 faxmailing up and running.</li> 188 189 <li>The Courier mail server includes a mailing list 190 manager, with fully automatic bounce processing.</li> 191 192 <li>You don't need a full-blown mail server? Courier 193 mail server's <a target="_top" href= 194 "http://www.courier-mta.org/imap/">IMAP server</a>, <a target= 195 "_top" href="http://www.courier-mta.org/sqwebmail/">webmail 196 server</a>, and <a target="_top" href= 197 "http://www.courier-mta.org/maildrop/">mail filter</a> are 198 available as independent packages that can be used with other 199 mail servers (as long as the other mail servers store mail in 200 <a target="_blank" href= 201 "maildir.html">maildirs</a>). 202 These sub-packages are assembled from the same source code 203 tree. The only difference is the top level makefile. Note: the 204 independent builds are not always in sync with the main the 205 Courier mail server build at any given time. They 206 follow their own schedule, and may include a slightly older, or 207 even more recent, code base! Over time, however, everything 208 always syncs together since all builds are assembled from the 209 same source code repository.</li> 210 211 <li>SOCKSv5 support. The Courier mail server can punch 212 through a SOCKS firewall to send outgoing mail. Receiving mail 213 through a SOCKS firewall is not yet supported. To use SOCKS you 214 need to install the Courier mail server's Socks 5 215 proxy client library.</li> 216 217 <li>PAM, LDAP, PostgreSQL (beta), or MySQL authentication. LDAP 218 authentication requires <a target="_blank" href= 219 "http://www.openldap.org">OpenLDAP</a> to be installed. 220 LDAP-based mail routing is also supported.</li> 221 222 <li>Gateway mail to/from UUCP (if compatible UUCP software is 223 separately installed).</li> 224 225 <li>Authenticated SMTP.</li> 226 227 <li><code>XVERP</code> and <code>XEXDATA</code> ESMTP 228 extensions.</li> 229 230 <li>DNS-based blacklists. Ability to exempt whitelisted IP 231 addresses from the blacklists.</li> 232 233 <li>Integrated mail filtering. An API is provided for 234 installing arbitrary external mail filters, and the system 235 administrator can selectively enable for any mail source 236 (ESMTP, UUCP, locally submitted mail) for filtering. Two 237 example mail filters are included - one written in C that uses 238 threads, and a Perl-based filter. The system administrator can 239 also enable the ability for individual mail recipients to 240 specify their own mail filtering rules, using a scripting 241 language (implemented by <code>maildrop</code>, see below). 242 Mail filtering is implemented as an integral part of the mail 243 server. Unwanted mail is rejected, and is not accepted by the 244 Courier mail server for delivery (the external mail 245 relay receives the error, and it becomes the external relay's 246 problem as to what to do with unwanted junk mail).</li> 247 248 <li>Partial ability to import <code>sendmail</code>'s aliases 249 file, but not all aspects of <code>sendmail</code>'s aliasing 250 is supported - like delivering to programs, for example. Still, 251 most simple <code>aliases</code> files should be usable.</li> 252 253 <li>Optional ability to import most of <code>Qmail</code>'s 254 <code>.qmail</code> files (Courier mail server uses an 255 almost 100% compatible local mail delivery instruction 256 format).</li> 257 258 <li>Most major components of the Courier mail server 259 can be installed in non-default directories, allowing extreme 260 customization for your particular environment.</li> 261 262 <li>You can set a maximum number of messages to deliver 263 simultaneously to the same host. This, in fact, is strongly 264 encouraged so that a single nonfunctioning domain does not take 265 up all available delivery slots. Rate limiting is implemented 266 in the main scheduler, and applies to any transport mechanism, 267 not just ESMTP.</li> 268 269 <li>Mailing list administrators can specify a backup relay and 270 have mail that's not immediately deliverable offloaded to a 271 backup server (this feature needs testing/feedback).</li> 272 </ul> 273 274 However, it is also important to note what the 275 Courier mail server does not have or will not 276 support: 277 278 <ul> 279 <li><code>.forward</code> files are partially supported. The 280 Courier mail server can import most basic 281 <code>/etc/aliases</code> files from <a target="_blank" href= 282 "http://www.sendmail.org">sendmail</a>, but sendmail's 283 <code>.forward</code> and <code>/etc/aliases</code> files are 284 simply not 100% compatible with the Courier mail 285 server's security model. Most <code>.forward</code> and 286 <code>/etc/aliases</code> files should be acceptable, but some 287 may not.</li> 288 289 <li><code>ETRN</code> is not, and will never be implemented. 290 It's a hack, and is functionally incompatible with the 291 Courier mail server's internal message dispatcher. If 292 a mail node does not have constant network connectivity, there 293 are better ways of arranging for mail transport than ETRN. The 294 transient mail node should download mail via IMAP, or maybe 295 even UUCP. </li> 296 297 <li>Workarounds for known defects in other mail software. The 298 Courier mail server will not accept mail with raw 299 8-bit characters in the headers, because they are illegal. 300 There are well-defined protocols that must be used to encode 301 8-bit text in mail headers. Non-compliant messages may result 302 in the Courier mail server itself issuing corrupted 303 delivery status notifications, or mishandling the message in 304 several other ways. Because of that corrupted mail will simply 305 not be accepted. Neither will the Courier mail server 306 deliver mail to domains with improperly-defined MX records, 307 even though other mail servers ignore the bad data. 308 Additionally, certain popular IMAP mail clients are known to 309 not work with the Courier mail server's IMAP server, 310 due to an improper IMAP implementation by the mail client.</li> 311 312 <li>Scripting language for rewriting mail headers. Mail 313 rewriting rules are hardcoded, and are expected to be 314 sufficient in most cases. If you have an unusual situation that 315 requires some oddball header rewriting, you'll have to 316 implement it yourself.</li> 317 318 <li>Support for mbox mailboxes in the POP3, IMAP, and webmail 319 components. They support maildirs only. There are plenty of 320 existing servers out there that read mbox mailboxes.</li> 321 </ul> 322 323 <h2>Requirements</h2> 324 325 <ul> 326 <li>A C++ compiler, <code>egcs</code> is recommended. Most of 327 the Courier mail server are written in C, but several 328 major sections are written in C++.</li> 329 330 <li>GNU make. Other <code>make</code>s may work, but that's not 331 guaranteed.</li> 332 333 <li>Either the GDBM or Berkeley DB library must be available. 334 Only certain versions of Berkeley DB API are supported, because 335 the Berkeley DB API often changes (tested with 2.4.14 and 336 1.8.5). GDBM is the recommended library.</li> 337 338 <li>Perl 5.</li> 339 340 <li>The file system must support FIFOs. At least the file 341 system that stores the mail queue must be able to support 342 FIFOs. The Courier mail server will not work with 343 AFS.</li> 344 345 <li>Filesystem domain sockets must be available.</li> 346 347 <li>Some optional components have additional dependencies - 348 notably the additional software required for faxmail support 349 (see above).</li> 350 </ul> 351 352 <h2>Additional information</h2>Here is a somewhat more detailed 353 overview of the Courier mail server's less prominent 354 features: 355 356 <h3>Upgrade path</h3>The Courier mail server can be 357 installed on systems that were previously running sendmail or 358 Qmail. Please note that the Courier mail server will be 359 able to support most major features of both servers, however the 360 Courier mail server is not, and will never be a 361 100%-compatible replacement for either sendmail or Qmail. The 362 Courier mail server does not implement several legacy 363 features of either MTA, and there are no plans to implement them 364 in the future. The key differences are: 365 366 <ul> 367 <li>sendmail</li> 368 </ul> 369 370 A local mail delivery agent, such as <code>procmail</code>, 371 should be used for maximum compatibility with sendmail. 372 373 The Courier mail server expects system mailboxes to 374 be in the users' home directories. If your system mailboxes are 375 all stored separately, in <code>/var/spool/mail</code> or 376 somewhere else, you'll need to use a local delivery agent such as 377 <code>procmail</code>. 378 379 The Courier mail server uses a filesystem lock on 380 mailbox files, The Courier mail server does not support 381 old-fashioned dot-locking. If you need dot-locking, use 382 <code>procmail</code> or <code>maildrop</code> (included). 383 384 <ul> 385 <li>Qmail</li> 386 </ul> 387 388 A configuration switch allows the Courier mail server 389 to read <code>$HOME/.qmail</code> files, however the 390 Courier mail server's implementation is not 100% 391 identical to Qmail's. The Courier mail server's 392 <code>aliases</code> file is also used to implement Qmail-style 393 virtual domains. A simple Perl script can be used to convert 394 Qmail's <code>control/virtualdomains</code> into 395 <code>aliases</code> entries. 396 397 The Courier mail server supports Maildirs 398 natively. 399 400 The Courier mail server can use the 401 <code>maildrop</code> mail filter as a local mail delivery agent. 402 <code>maildrop</code> is optional, but, if used, The 403 Courier mail server will take advantage of certain 404 <code>maildrop</code>-specific features which optimize local mail 405 delivery. 406 407 <h2>Mail filters</h2>The Courier mail server has hooks 408 for optional, site-defined, mail filters. You'll have to write 409 them yourself, though. The administrator-defined mail filters can 410 block the message from being accepted by the Courier 411 mail server (if messages comes in via SMTP, the SMTP server will 412 reject it). The Courier mail server can also be 413 configured to pause for a short period of time before attempting 414 to deliver a message. If the mail filter detects a slew of 415 duplicate messages coming in, the mail filter can block all 416 future copies, and manually bounce the handful of copies from the 417 queue. The system administrator can selectively enable filtering 418 for any individual mail source (ESMTP, locally submitted mail, 419 UUCP). The system administrator can also optionally enable 420 recipient-specified mail filters. With recipient-specified mail 421 filtering enabled, any local mail recipient can install an 422 arbitrary mail filter to selectively accept or reject mail based 423 on any criteria. 424 425 Currently the mail filtering API is not very well documented, 426 but it's there. 427 428 <h2>ESMTP extensions</h2>The Courier mail server 429 implements <code>AUTH</code>, <code>PIPELINING</code>, 430 <code><a href="courierdsn.html">DSN</a></code>, 431 <code>SIZE</code>, and <code>8BITMIME</code> extensions to SMTP. 432 The Courier mail server also includes a reference 433 implementation of the experimental <code>XVERP</code> and 434 <code>XEXDATA</code> extensions. 435 436 The Courier mail server is a closed mail relay by 437 default. The Courier mail server cannot be accidentally 438 configured as a completely open relay. A deliberate feat of 439 stupidity is required for that to happen. 440 441 <h2>ESMTP BOFH</h2>The Courier mail server does not 442 deliver mail to domains with broken MX records. The 443 Courier mail server also refuses to accept any mail with 444 a return address in a domain with broken MX records. 445 446 The Courier mail server can automatically blacklist 447 domains whose mail servers reject delivery status 448 notifications. 449 450 <h2>Header rewriting</h2>The Courier mail server will 451 rewrite headers and MIME-ify messages whenever appropriate. 452 Header rewriting logic is hardcoded in C, there is no header 453 rewriting language as in sendmail. An interpreted language 454 imposes a drastic speed penalty. The rewriting library is fairly 455 simple, and the the standard rewriting rules will do for most 456 situations. 457 458 The Courier mail server rejects messages with 459 badly-formed or missing MIME headers. The Courier mail 460 server rejects messages containing 8-bit characters in the 461 headers, or messages that contain 8-bit content, but do not have 462 the required MIME headers. Accepting malformed messages of that 463 kind can result in the Courier mail server itself 464 sending mail that violates the relevant RFCs, therefore 465 Courier mail server will simply reject 466 improperly-formatted messages. There are well-defined RFC 467 standards that explicitly spell out how mail containing 8-bit 468 content or 8-bit headers should be encoded, and those standards 469 will have to be properly implemented by anyone that wishes their 470 mail to be accepted. 471 472 <h2>Modularity</h2>Message scheduling, dispatching, and the 473 actual transport mechanism are completely modularized. Different 474 message transport mechanisms such as UUCP can be implemented in a 475 simple plug-in fashion, however some C coding will be required. 476 477 <h2>Message scheduling</h2>The Courier mail server 478 supports VERPs, multiple recipients per message, and 479 RFC1894-compliant delivery status notifications. 480 481 <h2>Load limiting</h2>You can set a maximum number of messages to 482 deliver simultaneously to the same host. This, in fact, is 483 strongly encouraged so that a single nonfunctioning domain does 484 not take up all available delivery slots. Rate limiting is 485 implemented in the main scheduler, and applies to any transport 486 mechanism, not just ESMTP. 487 488 <h2>Automatic restarts and garbage cleanup</h2>The 489 Courier mail server's scheduling engine restarts itself 490 automatically, on a regular basis. This helps with memory 491 fragmentation. The Courier mail server tries to restart 492 itself during periods of system inactivity. 493 494 <h2>Smart organization of the message queue and temporary 495 directories</h2>The Courier mail server automatically 496 creates subdirectories when necessary, and deletes them when 497 they're empty. When there's a sudden peak in the number of 498 messages added to the queue, directories used to store the 499 messages grow in size to accomodate the additional entries. On 500 many file systems, once those messages are deleted, the empty 501 slack space in the directory is not reclaimed, and actually slows 502 down subsequent directory operations. The Courier mail 503 server automatically removes empty directories, reclaiming the 504 slack space. 505 506 <h2>Smart installation layout</h2> 507 508 The Courier mail server's configuration script will 509 install the Courier mail server into 510 <code>/usr/lib/courier</code> by default. Everything will go into 511 several subdirectories there: the actual binaries, configuration 512 files, the mail queue, manual pages, and auxiliary support files. 513 Optional configuration switches allow pretty much every major 514 subdirectory to be relocated anywhere else. For example, the Red 515 Hat RPM package for the Courier mail server relocates 516 the configuration files to <code>/etc/courier</code>, the manual 517 pages to <code>/usr/man</code>, and the mail queue to 518 <code>/var/spool/courier</code>. 519 520 <h2>Mailing lists</h2> 521 522 The Courier mail server can implement both sendmail 523 and qmail-style address aliases. De-duping of sendmail-style 524 aliases is automatic. The Courier mail server source 525 distribution also includes a complete mailing list manager. 526 <hr /> 527 528 Copyright 1998-2020 Double Precision, Inc. 529 This software is distributed under the terms of the GNU General 530 Public License. See COPYING for additional information. 531 </body> 532 </html>