I run into this a lot using a "+" in my Gmail address. I like to use "username+site_name@gmail.com" for easy filtering, but there are still quite a large number of sites that do not accept this as valid, and it is extremely annoying.
Years ago, I must have signed up or given my email address to Urban Outfitters with a +uo. They happily continue to send me emails, but their unsubscribe form does not accept it as a valid address! I've tried reporting the bug, but now just send the messages straight to spam.
