By default, Gmail implements what some call plus addressing. The reason to use this is either filtering emails, or tracking the origin of spam.

Unfortunately, exactly the sites one might want to track do not read the RFC and tend to refuse such addresses.

A cryptic tweet by Joe Beda that I can no longer find hinted at the possibility of using a minus or period symbol instead of the plus.

How to do this:

• visit app details » email » default routing in Google Admin
• pattern match a regex such as username-.*@domain.tld (for minus addressing)
• modify such messages’ envelope recipient to yourself

Woohoo!