Linux System Administration

Configuring POP3 and IMAP Mail Delivery Agents

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Configuring POP3 and IMAP Mail Delivery Agents

In this section we’ll add email delivery agents to complement Postfix. Use the follow-

ing command on Debian to add an IMAP and a POP3 server:

# apt-get install ipopd-ssl uw-imapd-ssl

We’ve chosen ipopd-ssl to provide POP2 and POP3 mail delivery agents and uw-

imapd-ssl for IMAP. Don’t let the ssl suffixes fool you—both packages provide un-

encrypted services as well as encrypted ones. Standard IMAP uses port 143, and

POP3 uses port 110. The encrypted protocols and ports are POP3S (port 995) and

IMAPS (port 993).

Originally from the University of Washington, the ipopd-ssl package is now main-

tained by Debian. You just need to install it; it basically configures itself to use the

home mail directory that exists on a mail server like the one we set up in Chapter 4.

ISPs continue to use POP3, but it is rarely used in enterprises.

uw-imapd-ssl provides an IMAP server. Although it requires more disk space, IMAP

is superior to POP because it leaves mail on the server and allows users to view mes-

sages from any location that has Internet access and a mail client. We don’t know of

any current mail clients unable to understand IMAP, so most mail users will want to

use it.

You can also provide webmail on your mail server using SSL (https), making it conve-

nient for users to access their email from a web browser.

In our configuration, users need standard Linux accounts on your

email server, even though they read their mail with an email client on

another system. Postfix usually allows local delivery inside a domain,

but it requires backend relaying (as previously discussed in the section

“Configuring Postfix with SASL to authenticate users without

accounts”) if users are outside the domain.

uw-imapd has advantages and disadvantages. On the plus side, it uses Unix-style

mbox email storage, which maintains all of a user’s mail in a single file in his home

directory. You will also find this service easy to administer.

On the minus side, uw-imapd does not allow virtual users or those without shell

accounts and home directories to access mail. In addition, many administrators

don’t like the simple mbox storage format, preferring the more hierarchical maildir

format. As a single-file format, mbox allows only one application to access it at the

same time, which requires file locking and might slow the system under heavy load.