My usecase: I have one email account with multiple aliases.

I added it to iOS mail with the main alias. I registered to my bank using a secondary address, a more "neutral toned", also one I'd expect to be sold to data brokers, and would be willing to abandon easily if started to get too much spam...

Now the banking app is pretty... ripe for improvement.

I report a bug in the app, and the way it happens: it throws up an email editor, with debug info attached to a partially pre-filled form mail. However if its sender is not the alias I set up at the bank, it will get rejected, and will have to do lots if identification turns etc...

Apple mail does not let you set up aliases for an account, I'd need to log in twice to the same account to be able to do this. Also apple mail is a piece of ... modern outlook, to be really rude, so I'm interested in alternatives.

In iOS Mail aliases aren't available only with the email providers that you can pick directly on the Add Account screen. But if have an account with someone else and had to add the IMAP/SMTP servers manually, you can add an alias in Account Settings -> Email -> Add Another Email.