Mac Forum / Applications / Eudora / June 2009

Compacting occurs automatically, IIRC when 50% of the file space is unused.

During compacting, not only is the mailbox (message file) rewritten,
but also its "table of contents" (TOC), from which is displayed
the "mailbox window."

"Deleting" messages consists of removing their TOC entries,
so that deleted messages are no longer listed,
but the message remains in the mailbox file,
much as "deleted" computer files remain stored on disk,
while only their "directory" entries are removed.

It would appear that what went wrong in your case was that
the TOC was found to be either missing, corrupt, or out of date
(not as recently updated as the mailbox itself),
hence was rebuilt to re-index everything still in the mailbox file,
which would include anything deleted, moved, or updated
since the last compacting.

The TOCs of built-in mailboxes (e.g. In/Out/Trash) are held in
RAM memory all the time, which exposes them to increased chance
of getting damaged, particularly if one does not trim down these mailboxes
to contain only a modest amount of mail, by moving old mail instead,
to user-created mailboxes.

One has a choice to either use separate files for mailbox TOCs,
or to use Mac "Resource forks" to store them.  It seems possible
that using resource forks, which is now the default,
might be less robust a method than the original "separate TOC files" method.

You do not have to do it yourself, since eventually it will be done for you,
although you may always elect to do it, whenever you want,
either by clicking any individual mailbox size display,
or with the Option key to compact all mailboxes
(see page 312 of version 6.2.4 user manual)

Manual compacting does not help prevent TOC corruption,
although it may result in fewer "resurrected" deleted messages
if and when any TOC may ever need to be rebuilt, plus smaller backups
(if you or Time Machine make backups).

Keeping the size of built-in mailboxes small is what will help prevent
the need to rebuild damaged TOCs, as well as possibly changing back
to the original "separate TOC file" method.

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