Conflicting Mail has both IMAP and POP

A friend called me earlier today because his iMac (2009) had run out of space on the main disk.

He had watched it counting down free space while he was doing nothing.

After digging about and clearing out Trash, I found that his Apple Mail was 136GB in size and that it had two accounts defined. Both were his email address; once defined as POP and another as IMAP. When Mail was open and in the foreground, one could see the "twirling clock face" (busy icon) beside both.

I deleted the IMAP account (he'd said he has nothing important in his mail) and the newly freed up space stopped counting down. So that was a start. I then thought we could reboot. However, that was held up because Apple Mail couldn't shut down. I suspect it may be unwinding whatever it had done. I've now left it for the day and it's sitting there, doing lots of IO but not shutting Apple Mail.

Tomorrow, if Apple Mail hasn't shut down I plan a Forced Quit, followed by a reboot and Verify Disk.

• Does that sound reasonable?
• Is there a mode of starting Apple Mail where it might rebuild its mail?
• After this, any suggestions on where I might find files that could be trashed?
• 136GB for Mail sounds gigantic. (My own email, which goes back 9 years, is 8GB in size).

Your suggestions will be welcome.