This will most likely do the job:
Cribbing from Chapter 7 "Managing Mail" in the PDF manual
(available at any time via Help -> User Manual):
# Maintaining Mailboxes [page 66]
## Rebuilding Mailboxes
Rebuilding mailboxes is occasionally necessarily if a mailbox
has become corrupted. If you have been advised to rebuild your
mailboxes by technical support:
* Quit Mailsmith, if it is running.
* Re-launch Mailsmith while holding down the Command and
* Mailsmith will present the Rebuild Mailboxes dialog. [[image]]
* Click "Rebuild" to start the rebuild process.
If you do not wish to rebuild, click "Quit" to quit Mailsmith,
or "Continue Startup" to launch Mailsmith without performing a rebuild.
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