I used this wonderful trick to sign up for my government issued eID (it was something else but works for explaining). What they decided to do is to simply remove the + and don’t let me know about it.

[email protected] thus became [email protected]

I tried logging in, resetting passwords, nothing worked. I had to go to the authorities and make a written request to allow them to interrogate the database by the equivalent of my social security number, and that’s when we realized they just stripped the +.