Email > Advanced use >
Compact Email Mailboxes
If your email application supports it, you should
periodically compact your mailboxes.
Some email programs like Thunderbird
and Eudora maintain your mailboxes as efficiently as possible by
temporarily recording updates
the end of each mailbox file, instead of performing
edits like additions, moves, and changes within the mailbox each time.
programs then have an option to "compact" the file to work in all the edits at
one time (Thunderbird:
Folders; Eudora: Special / Compact Mailboxes).
If your computer crashes in the middle of a mailbox operation, and
your mailboxes haven't been compacted in awhile, the settings
that keep track of the updates can get lost. When this happens, you
can find that deleted messages have been restored, messages
moved are still there, and other updates since the last compact
operation have been lost. While this is rare, to protect against it
and keep your files in good order you should compact your mailboxes
about once a week, or
after any significant
set of changes. Compacting your mailboxes also saves some disk space,
and could make your program run a bit faster.