[zlynx]

I like Thunderbird and Mutt.

If your system can't handle and categorize IMAP folders of 50,000 messages and 500 new emails per day, don't bother on my account.

Have good GPG support.

Search is important. My cyrus imap server can find good search results in a couple of seconds out of 15 GB of email. You'd have to at least match that.

Make sure I can send messages in plain text with formatting that is exactly how I wrote it or pasted it.

But on the other hand, I do need to be able to read HTML email. I just won't write it.

[RomaTesla]

Great feedback, thank you! Which OS do you use? Are most of those 500 emails promotional, or do you communicate via email a lot?

Thanks in advance!

[zlynx]

I use Linux, Windows and Android.

Mailing lists. Then probably promotional. Notifications from things like Facebook, LiveJournal, Patreon, various web forums, etc. Then probably status reports from various scripts. The least amount of email is from people I actually talk to.