Why does reconstruct -m not work?

From the reconstruct(8) man page:

−m     NOTE: CURRENTLY UNAVAILABLE
             Rebuild  the  mailboxes file.  Use whatever data in the existing
             mailboxes file it can scavenge, then scans all partitions listed
             in the imapd.conf(5) file for additional mailboxes.

Reconstruct is currently unable to rebuild the mailboxes db, and comments on the mailing list indicate this ability will not be added; at least, not any time soon.

The solution here is to make sure that you back up your mailboxes db, preferably including a plain-text copy of it. Please see Backups for more information.

If you do find yourself with a corrupted mailboxes.db, there are a few things you can try. The first is to see if db_recover can recover your database. If that doesnt' work, there should be backups in $CONFIGDIRECTORY/db.backup1 and $CONFIGDIRECTORY/db.backup2 that may be OK.

If you're using Simon Matter's RPMs, plain-text copies of the mailboxes database should be being generated and saved in /var/lib/imap/backup. Try rebuilding the db from one of those using ctl_mboxlist(8). Alternately, try asking the mailing list for help.