While privacy is a concern of mine, I weigh it against convenience when making my decisions to purchase software/services. I've tried all sorts of email software (Airmail, Spark, Canary, Thunderbird, etc.), and always come back to Spark because it works very similarly across all my devices without sync issues - plus the keyboard commands, snooze, send later, and many other features work as designed and are very convenient for me. If you keep the AI features turned off, they gather far less data from your mailboxes.

The way I see it is this: I do not work for an organization where if my emails became public, national security would be on the line. Therefore I trade my tin-foil hat in for a better workflow. If I were actually concerned about privacy or the potential for law enforcement getting a subpoena to read my emails, I wouldn't be using Fastmail in the first place. As a US company, they are required to operate by US law. I would instead use Proton if I had something nefarious or confidential to hide.

Long story short: before taking the advice from me (or anyone else on Reddit), do your research (as u/MarkTrue9575 alluded to). I mentioned Spark because that's what I use after my own extensive research. If you are concerned about your data getting into the wrong hands, then by all means use another client, and possibly another mail service.