I have a few requirements for an email client. My primary email is through the same company I use for webhosting (Opalstack in my case). I'd prefer if other com

I have a few requirements for an email client. My primary email is through the same company I use for webhosting (Opalstack in my case). I'd prefer if other companies didn't read my email, so I don't use gmail/outlook/similar via IMAP/POP or forwarding or anything like that. I also only send plaintext email. This sounds like it should be easier than sending HTML, but many mobile email clients only send HTML or "rich emails". This includes the gmail! (This used to be possible in earlier versions of google's phone email apps. Maybe it was Inbox? I don't remember now. But being unable to send plaintext email from gmail on mobile is what caused me to move away from gmail). Now I use mutt on my own computer, K9 on mobile, and if absolutely necessary Roundcube webmail. (I do not particularly like Roundcube, but I almost never actually have to use it). Mutt is wonderful, though I found it highly nontrivial to initially use. K9 is fine. One feature of K9 I like is that it has a surprisingly capable search capability given that I use it purely over IMAP --- it dispatches a search to my mail server and parses the results.