Fastmail supports "subdomain addressing" out of the box. So if the email address is ... user@domain.com ... then you can use ... <anything>@user.domain.com ...

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