In a nutshell, mailbox3 has gone kaboom. From the mysql log:

InnoDB: stored checksum 1643286861, prior-to-4.0.14-form stored checksum 2860349746
InnoDB: Page lsn 0 10615170, low 4 bytes of lsn at page end 10615170
InnoDB: Page number (if stored to page already) 155,
InnoDB: space id (if created with >= MySQL-4.1.1 and stored already) 15
InnoDB: Page may be an index page where index id is 0 63
InnoDB: (index PRIMARY of table mailbox3/mail_item)
InnoDB: Database page corruption on disk or a failed
InnoDB: file read of page 155.
I know it is mailbox3 because simple queries there cause it to crash:

mysql> show tables;
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 2
Current database: mailbox3
The other mailbox databases appear to be fine. Before the obvious is pointed out, there's no mysqldump of that database , so it cannot be restored. However, the message stores and such all appear to be just dandy.

Dropping the database and re-initializing is easy enough, since the schema can be taken from another mailbox. While obviously appointments and address books and such are lost, it appears to be possible to rebuild the mail_item table (or enough of it) from the messages themselves to make the mailbox functional again. A little perl, a little patience, new table.

The hitch in that plan is that I can't seem to find any doc/discussion on just what comprises the fields in the table. From looking at another mailbox, obviously rows of type 6 are address book data, 5 and 4 are message data, type 1 is data on various folders. The various metadata bits and such are, of course, not obvious.

I could not find mention of any of this in architecture docs, though if I missed it just point it out and I'll go read some more.