[climb_stealth]

Is this what you have in mind? See the section on mirrored domains.

https://www.fastmail.com/help/receive/domains.html#mirrored

[tmpxgdqrcKFuG]

Yes! That's exactly what I was thinking. I think I like the way fastmail does it more than gmail too.

[climb_stealth]

For what it's worth I have found Fastmail's documentation to be really good. Whenever I had to look up things like configuring email clients or their spam rules I have been pleasantly surprised.