One Fastmail alias can be "(asterisk)@domain.tld", by the way, so you can send and receive emails to every address possible. When you send an email from the (asterisk) alias, you can type whatever you want in the from address box. I use their DNS as my primary DNS for all my domains so I can email to/from all of them easily like this.

(asterisk) means the character itself, I can't figure out how to type one on this site though.

Edit: this wasn't a feature 3 years ago, I only discovered this 4 months ago when I added a few domains to my account and they added the (asterisk) aliases for those automatically.