[webmobdev]

Some services do not accept email with a "+" in it.

[PrettyPastry]

Some services even accept it to create an account, but not to log in.

One never let me change my email or password when I used the +.

[KennyBlanken]

Postfix allows defining any character as a VERP separator.

OP also could have just used a regex in the virtual file.