There was an email account that was rather large (40G). The emails up until a certain date were archived, and they were to be removed after the user had time to keep what they wanted.

I had deleted the Maildir/archive2015 file which was 40G or so, and there is still 40G in the dovecot.index.cache file. I looked through the dovecot manpages looking for a way to clear that, but I didn't find anything.

If I if rename dovecot.index.cache to dovecot.index.cache.old, will dovecot create a new dovecot.index.cache, thus allowing me to delete the 40G renamed file?

I usually just try out things like that. I think I'd
1. stop dovecot
2. mv the cache file
3. restart dovecot
...and see what happens. If dovecot creates a new cache file (and I think it will), then you'd be ok to remove the old one

Before posting this, I did a search for "reset dovecot cache" and found this

Thank you for that link Sean. Dovecot docs say Dovecot will recreate and index files that are missing when the user checks their mail next. I did every search but 'reset'. I will try to delete the index.cache to see what happens, and if all works, I'll close this. It's a very busy mail server, so I am apprehensive about stopping it for one mailbox.

I deleted the dovecot.index.cache file, and I watched the logs. The user was able to login without issue, and Dovecot created a new dovecot.index.cache file.