Fastmail also supports subdomain aliasing (where messages sent to <anything>©<username>.<domain> go to the <username>©<domain> inbox). This is less obvious to mass emailers that GMail + aliasing, and I don't think it can be detected alogrithmically in the way a + in the username on gmail.com can.
I normally sign up for services with an email that attests to the account's original intent, e.g. I buy cat food using chewy.com@<username>.<domain>.