> They want to spam you later (or sell your address to spammers) and have you not know where they got the address. Email +suffixes are easy enough to strip off

> They want to spam you later (or sell your address to spammers) and have you not know where they got the address. Email +suffixes are easy enough to strip off post-collection if you're aware of them. If they wanted to spam, they wouldn't have to disallow +'s in the validation regex. They must have some other reason for disallowing them, which may be as innocent as poor awareness of them. Personally, I used Fastmail with my own domain and have abandoned using +suffixes because they're too unreliable. Fastmail has several options for single-use email addresses that should be difficult or impossible for recipients to decode to the main address.