Using a plus sign is subaddressing [1] and most ESPs[2] will route to the main address ( multiple@addre.es) . So you can use use multiple+email@adress.es, multiple+xyz@adress.es and both will route the email to you.
In my experience most SaaS apps do not filter this out and allow re-sign ups with sub-addresses.
Gmail has an additional behavior that dot character is ignored in local component of the address . multiple@gmail.com, mult.iple@gmail.com mult.ip.le@gmail.com all route to the same inbox as well.
This is not true (anymore?). I have a rather unfortunate exact naming collision with a family member. They use the full name without dot for the local gmail component, I use a dot between the first and last name.
Two or three mails have been misplaced in a decade.
It would be feasible to change something like that without breaking security now.
Google can hardly start allowing/routing a new account for first.last@gmail.com when you were getting it for years even though your account is firstlast@gmail.com and sensitive communication like say from your bank would routed there.
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