In the administration/user screen, using Generic UI, an e-mail address with a plus sign is rejected as invalid.
This is not the expected behaviour. Please refer to RFC or Email address - Wikipedia (special characters as !#$%&'*±/=?^_`{|}~ are valid.
There are some other “non-compliances” in the current validation (example Jhon..Doe@myDomain.com is accepted, while two or more consecutive dots should only be accepted when the local part is quoted.)
The plus sign is quite important as it is used, amongst other, by GMail and optionnally Postfix (see recipient_delimiter option), to auto tag mails and allow transparent aliases.
To clarify, a mail to Jhon.Doe@gmail.com and Jhon.Doe+cuba.platform@gmail.com will both arrive to the same account (Jhon.Doe@gmail.com) but the second one will be tagged with “cuba.platform”.
Kind regards
Thierry