This is why we allow subdomain magic at FastMail, so that if you wanted to use myname+foo@fastmail.com you can use foo@myname.fastmail.com as well, and it works

This is why we allow subdomain magic at FastMail, so that if you wanted to use myname+foo@fastmail.com you can use foo@myname.fastmail.com as well, and it works fine. Everyone accepts that form.