See MailboxFormat/mbox for a complete description of how Dovecot has implemented mbox support.
In many systems the user's mails are by default stored in /var/mail/username file. This file is called INBOX in IMAP world. Since IMAP supports multiple mailboxes, you'll need to have a directory for them as well. Usually ~/mail is a good choice for this. For installation such as this, the mail location is specified with:
# %u is replaced with the username that logs in mail_location = mbox:~/mail:INBOX=/var/mail/%u
It's in no way a requirement to have the INBOX in /var/mail/ directory. In fact this often just brings problems because Dovecot might not be able to write dotlock files to the directory (see below). You can avoid this completely by just keeping everything in ~/mail/:
# INBOX exists in ~/mail/inbox mail_location = mbox:~/mail
See MailLocation#indexfiles for full explanation of how to change the index path. For example:
mail_location = mbox:~/mail:INBOX=/var/mail/%u:INDEX=/var/indexes/%u
Make sure that all software accessing the mboxes are using the same locking methods in the same order. The order is important to prevent deadlocking. From Dovecot's side you can change these from mbox_read_locks and mbox_write_locks settings. See MboxLocking for more information.
Often mbox write locks include dotlock, which means that Dovecot needs to create a new "<mbox>.lock" file to the directory where the mbox file exists. If your INBOXes are in /var/mail/ directory you may have to give Dovecot write access to the directory. There are two ways the /var/mail/ directory's permissions have traditionally been set up:
World-writable with sticky bit set, allowing anyone to create new files but not overwrite or delete existing files owned by someone else (ie. same as /tmp). You can do this with chmod a+rwxt /var/mail
- Directory owned by a mail group and the directory set to group-writable
You can give Dovecot access to mail group by setting:
mail_privileged_group = mail
NOTE: With deliver the mail_privileged_group setting unfortunately doesn't work, so you'll have to use the sticky bit or disable dotlocking completely.
NOTE 2: Older Dovecot versions used mail_extra_groups setting. Its usage could result in security problems in some setups.
The settings below are related to mbox performance. See MailboxFormat/mbox for more complete description of what they do.
mbox_lazy_writes=yes (default): Metadata updates, such as writing X-UID headers or flag changes, aren't written to mbox file until the mailbox is closed or CHECK or EXPUNGE IMAP commands are sent by the client. The mbox rewrites can be costly, so this may avoid a lot of disk writes.
mbox_dirty_syncs=yes (default): Dovecot assumes that external mbox file changes only mean that new messages were appended to it. Without this setting Dovecot re-reads the whole mbox file whenever it changes. There are various safeguards in place to make this setting safe even when other changes than appends were done to the mbox. The only downside to this setting is that external message flag modifications may not be visible immediately.
mbox_very_dirty_syncs=yes (not default): When opening mbox file that has been changed externally, don't re-read it. Otherwise similar to mbox_dirty_syncs=yes.
mbox_min_index_size=n: If mbox file is smaller than n kilobytes, don't update its index files. If an index file exists for it, it's still read however.
Only /var/mail/ mboxes
With POP3 it's been traditional that users have their mails only in the /var/mail/ directory. IMAP however supports having multiple mailboxes, so each user has to have a private directory where the mailboxes are stored. Dovecot also needs a directory for its index files unless you disable them completely.
If you really want to use Dovecot as a plain POP3 server without index files, you can work around the problem of not having the per-user directory:
Set users' home directory in userdb to some empty non-writable directory, for example /var/empty
Modify mail_location setting so that the mail root directory is also the empty directory and append :INDEX=MEMORY to it. For example: mail_location = mbox:/var/empty:INBOX=/var/mail/%u:INDEX=MEMORY
Note that if you have IMAP users, they'll see the /var/empty as the directory containing other mailboxes than INBOX. If the directory is writable, all the users will have their mailboxes shared.