> I'd move the oldest 20-30 thousand to a different folder, and see if the slimmed-down inbox solves the problem.
Early on I deleted a few hundred emails on the assumption that it was a volume problem, but Mail still went Unexpected. Mail is snappy, no choking.
< ...have an Apple store do the wipe and OS install, rather than slog through the download/update/upgrade swamp. (Make an appointment if it's a busy shop.) They should be able to do a one-shot installation of El Capitan 10.11.6.
If she thought the Apple store would help I would never have been asked to get involved. The last time she was there the genius refused to help, and told her to come back when she had a newer Mac.
The HDD was replaced by the SSD after El Capital was installed, so it is close to a clean/new install.
> I would certainly look at archiving some of the older emails to remove them from the server
Archiving on the Mac or at gmail?
She started to Delete or Archive the oldest emails early on to reduce the number of Inbox emails. After doing hundreds she complained that Unexpecteds were still happening so I deleted the cache, and 38,000 re-appeared in Mail's Inbox. I would have to verify there are no duplicates between Inbox and Archive before saying it made no difference.
Mail is functioning on the HDD boot with no Unexpecteds to be seen.
Next weekend I will clone the HDD to the SSD. If Unexpected happens again the SSD will be replaced.