Goodbye MS Exchange: Good Linux Email Servers - page 2
Say Good-Bye to Jabba
POP3Post Office Protocol version 3. This is an application-layer standard used by MUAs to retrieve email. POP3 is the simplest to administer and has the lowest overhead; a modest PC can serve hundreds of users. Users retrieve messages with their MUAs and store them on their computers, and then they are deleted from the server. Many MUAs offer a "leave messages on the server" option so they are not deleted, or are deleted after a specified period of time. Users who log in from multiple locations will have a hard time keeping their local mail stores synchronized, and sent messages will only be on the machines they were sent from. For roaming users IMAP or Webmail are better options. Some examples of good FOSS POP3 servers are Dovecot, qpopper, Courier, and Cyrus. (See RFC 1939, RFC 2449 and RFC 1734.)
IMAP4Internet Message Application Protocol version 4. Messages remain on the server until the user deletes them. You can access an IMAP server from any MUA that supports IMAP, on any computer, and it will always look the same, with the same messages, directory structure, and even sent messages. An IMAP server needs more horsepower and considerably more storage than a POP3 server. Dovecot, Courier, and Cyrus are good IMAP servers. IMAP support varies among the different MUAs; KMail, Claws, and Thunderbird have good IMAP support. (See RFC 3501.)
WebmailWebmail is like POP3 in a Web browser. It's slow and has limited user-customization options. But for simple email reading and writing it's portable and low-overhead for the server. Squirrelmail and OpenWebmail are both good Webmail servers. These run on top of an IMAP server and you'll need a Web server such as Apache or Lighttpd.
TLS/SSLYou'll also want TSL/SSL to encrypt logins, which is supplied by OpenSSL. smtp-auth is for authorizing SMTP traffic. Usually this is for authorizing one SMTP to another, though you can also use it for individual users.
Internet VerminSpamassassin on the server is a nice thing to reduce the burden on user's inboxes, and ClamAV is a decent FOSS malware scanner. Postfix has a number of its own excellent internal filters and header checks for blocking nasty stuff.
Isn't FOSS great? All of these applications are Free Software/Open Source, high-quality, and use open standards. There are no tricks or traps, no proprietary gotchas, no lock-in. The standard mail store formats are mbox, maildir, and MH. Mbox stores all messages in a single file. Maildir and MH store each message in its own
directory file. A single file is a little more vulnerable to corruption. All are plain-text so they are easily recoverable without special tools, easy to backup and easy to restore. MUAs and MTAs can use different mail store formats; they don't have to match each other.
In Part 3 we'll take a look at the better Linux groupware servers.
ResourcesTesting Linux Mail Servers with OpenSSL
Troubleshooting Linux Servers with telnet
Exim Internet Mailer
The Apache SpamAssassin Project
Carla Schroder is the author of the Linux Cookbook and the Linux Networking Cookbook (O'Reilly Media), the upcoming "Book of Audacity" (NoStarch Press), a lifelong book lover, and the managing editor of Linux Planet and Linux Today.