You mean you can send emails to <user>-<tag>@<domain> and it'll be delivered to <user>@<domain> account (assuming MX for <domain> is pointing at fastmail)?

Is there anything you need to enable to allow this? I just tried, and the email got bounced back.

Or are you talking about email aliases? As in, you register <user>-<tag>@<domain> first before sending emails to that address?