Impossible to subscribe with email with plus sign in it

[paskal]

When I try to subscribe [email protected] for email notifications, frontend doesn't pass plus sign to backend:

bad to address "user [email protected]`": 501 Invalid command or cannot parse to address

Here is the reference for valid email regexp, @akellbl4 can you please make sure that they can be passed to backend properly?
It would be nice to write a test case for that as well.