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You should just be able to add the other aliases in the settings for that account. I haven't personally used it but the option is there.

I don't see that option in the iOS Mail settings. Can you point me to it?

Edit: here it is: https://www.fastmail.com/help/clients/iphone-aliases.html I'm going to give it a try now. Thanks!

Have been using it for years, it works great!