Mac Mail suddenly crashing (after I send an email w/ attachment)

I'm using an older OS, 10.13.6, on an iMac. Mac Mail functioned perfectly for years, even through an upgrade 18 months ago to a new host (Proton) and conversion from POP to IMAP. Two or three weeks ago, it started to crash -- but in no predicable way, except when it happens, it's after sending an email with an attachment. Not every time, though, not even every few times, but often enough.

I've run Disc First Aid multiple times, and my Mac guy ran Disc Wizard last week; it found and fixed a number of directory issues. Didn't fix the problem though.

I've seen other posts here that suggest running in Safe Mode or disconnecting from Internet; that's not really a solution for an email problem! I cannot think of anything system-wise that might've changed around the time this began occurring.

Any suggestions, at least short of upgrading my OS/hardware, would be appreciated. That's inevitable but I'd prefer to delay it (as I'm a graphic designer and rely on PostScript Type 1 fonts a great deal -- one of several reasons). Thank you.