@armin @mwl Mail on #apple is a very difficult thing, I'm in the middle of that myself
On #macos : several options, I find #thunderbird is the best you can get.
On #iOS: to my knowledge there is no #opensource option for a mail #IMAP client. Which is crazy! Only options are tied to a specific (and external, so not for you) provider: #tuta , #protonmail , in my case #mailfence .
I'm afraid that your only option on #iOS is the default #mail app...
#selfhost and #apple clearly don't seem to be good friends...