Fastmail supports "subdomain addressing" out of the box. So if the email address is ... [email protected] ... then you can use ... <anything>@user.domain.com ... without setting up any extra aliases, Fastmail will just handle it as if it's for [email protected].

And you can also reply from such addresses without extra configuration. On the desktop I use MailMate as my email client, which works great with dynamic aliases. The win is that you don't have to configure anything extra when signing up for some online service.

https://www.fastmail.com/help/receive/addressing.html

You can do this in Google Suite btw, as you can configure a forwarding rule. The problem with Google Suite, last time I tried, is that they no longer want to sign emails with DKIM for such dynamic aliases, so you can receive emails just fine, but sending emails from such addresses is a problem if you have SPF/DKIM domain rules.

That's good to know. I've only just switched 48 hours ago, so this is something I'll look at.

I had them handle my DNS for that domain so I'm assuming this won't require any extra setup.

This is what I do, for both Fastmail and replicated for my own email domain.

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.