@Plusplus Most often than not, the e-mail "syntax" validations that block it are Javascript and client-side only. If you're able to register it with NoScript or Javascript disabled, you're generally also able to log-in with the registered "+"-tagged email. I don't remember which service now, but there was one case where I could register it tagged, but the backend automatically trimmed the tag away, and I noticed it because the confirmation email came untagged.
Mismatch
@Plusplus Most often than not, the e-mail "syntax" validations that block it are Javascript and client-side only. If you're able to register it with NoScript or Javascript disabled, you're generally also able to log-in with the registered "+"-tagged email. I don't remember which service now, but there was one case where I could register it tagged, but the backend automatically trimmed the tag away, and I noticed it because the confirmation email came untagged.