Dovecot as a POP3 server
Dovecot was primarily designed to be an IMAP server, so although it works fine as a POP3 server, it's not really optimized for that.
The main problem with Dovecot's POP3 implementation with Maildir is how to get messages' size fast enough. The POP3 specification requires that the sizes are reported exactly, not just approximately. This means that linefeeds must be counted as CR+LF characters. Normally with Maildir the linefeeds are stored as plain LF characters, which means that simply getting the file size would produce the wrong POP3 message size. Some Maildir POP3 servers do this anyway and violate the POP3 specification.
Dovecot returns correct message sizes by reading the entire message and counting the linefeeds correctly. After this is done, the "virtual size" is stored into dovecot-uidlist file and future calculations can be avoided by simply looking up the cached value.
You can also avoid the initial message size calculation by storing the size directly into the filename. You can do this by appending ,W=<size> at the end of the base filename. For example 1199932653.M583975P6568.host,W=2211:2, is a file whose virtual size is 2211 bytes (and real size somewhat smaller). Note that this must not be done for existing files, only to newly delivered mails.
If Dovecot's LDA is used, dovecot-uidlist and the index files are updated upon message arrival, therefore there will be no message-size performance issues.
pop3_fast_size_lookups=yes setting in v2.0.5+ uses the virtual message sizes when they're already available, but fallbacks to using the physical message sizes (violating POP3 specifications, but then again a lot of POP3 servers do that).
Index files are quite useless if your users don't keep mails in the server. They get first updated when the POP3 session starts to include all the messages, and after the user has deleted all the mails, they again get updated to contain zero mails. With this kind of a session the index reads and writes could have been avoided if the index files had just been completely disabled.
You may want to try how performance changes if you disable indexes for POP3 users. You can also try preserving indexes but try different values for mbox_min_index_size setting.
Do not disable indexing if there are users that do not delete messages after downloading them. Also, if you use Dovecot LDA, indexes may be helpful to have fast access to the message sizes.
By default Dovecot allows multiple POP3 connections to the same mailbox. This is (was?) especially useful for dialup connections which die in the middle of the download, because the half-dead connections won't keep the mailbox locked.
Setting pop3_lock_session=yes makes Dovecot lock the mailbox for the whole session. This is also what the POP3 RFC specifies that should be done. If another connection comes while the mailbox is locked, Dovecot waits until the locking times out (2 minutes with Maildir, mbox_lock_timeout with mbox). In future there will be a separate pop3_lock_timeout setting which allows timing out sooner.
By default when a message is RETRed, \Seen flag is added to it. POP3 itself doesn't support flags, but if the mailbox is opened with IMAP (eg. from webmail) it's shown as seen. You can disable this (to get better performance) with pop3_no_flag_updates=yes.
POP3 client workarounds
pop3_client_workarounds setting allows you to set some workarounds to avoid POP3 clients breaking with some broken mails.
UIDLs are used by POP3 clients to keep track of what messages they've downloaded, typically only if you've enabled "keep messages in server" option. If the UIDL changes, the existing messages are re-downloaded as new messages, which the users don't really appreciate.
Dovecot supports multiple different ways to set the UIDL format, mostly to make migrations from other POP3 servers transparent by preserving the old UIDL values. See Migration for how to set the UIDLs to be compatible with your previous POP3 server.
For new POP3 servers, the easiest way to set up UIDLs is to use IMAP's UIDVALIDITY and UID values. The default in Dovecot v1.1+ is:
pop3_uidl_format = %08Xu%08Xv
Another good default is to use the message's global UID:
pop3_uidl_format = %g
However, note that GUIDs may not be unique, as the GUID does not change when a message is copied. (While copying is not possible using only POP3, it can be done using IMAP, Sieve, or doveadm.)
Some formats, such as the previous default %v.%u, seem to have problems with Outlook 2003.
MD5 UIDL format (mbox-only)
pop3_uidl_format = %m
This works by getting the MD5 sum of a couple of message headers that uniquely identify the message. The one good thing about MD5 format is that it doesn't rely on the IMAP UID or UIDVALIDITY value. This allows you to modify the mbox files in ways that Dovecot doesn't like, without causing the UIDLs to change. For example:
- Inserting messages in the middle of mbox files (eg. restoring mbox files from backups can cause "Expunged message reappeared" errors)
- Reordering messages inside mbox
Other random problems causing UID renumbering (although you should figure out why they're happening)
The MD5 summing method however doesn't work well if you receive two identical messages. Usually the MD5 sum is taken from these headers:
- The first Received: header
- Delivered-To: header
Normally there won't be a problem, because the MTA adds a unique identifier to the first Received: header. If the same message is sent to multiple users in one delivery, the Delivered-To: header is still different, making the MD5 sum different.
Except the MTA can be configured to support aliases, so for example sending the mail to both root@ and webmail@ aliases causes the message to be delivered to the same user, with identical Received: and Delivered-To: headers. The messages really are identical, so their MD5 sums are also identical, and that can cause some POP3 clients to keep downloading the messages over and over again, never deleting them.
To avoid this, there's also a 3rd header that is included in the MD5 sum calculation:
- X-Delivery-ID: header
If you use Dovecot's deliver or IMAP APPEND and pop3_uidl_format = %m, it always appends the X-Delivery-ID: header to saved mailbox. Any existing X-Delivery-ID: headers in the saved mails are dropped.