RFC5322 allows the usage of + in e-mails.
Some mail services interpret + such that all mail that is sent to user+foo@example.com,
user+bar@example.com ends up in user@example.com mailbox.
This helps managing your inbox, is extremely useful if you want to detect a service that sells user
e-mails to spammers and just keeps yourself more secure.
Some services don’t support this practice, though.
Here's a list of webservices that has no idea how to validate an email address properly or just prohibits this feature for whatever reasons.
Hall of shame if you will.
