Is the best, free (not necessarily open source), email desktop client still Thunderbird?
I personally migrated to Evolution at a particularly frustrating moment relatively recently, and it's been less painful for a lot of things. It has built-in calendar and PGP support, which certainly help. Unfortunately searching through message bodies is unusably slow.
I don't know whether my standards are unreasonably high, but several colleagues have expressed similar views on both.
I find the search in Thunderbird to be amazing. I have archives going back 20+ years and it searches them instantly.
Self-hosting email with dovecot, if that makes a difference.
Also a unified calendar is out of the question.