It's not really all that standardized. The use of a '+' character to indicate an alias or label is merely convention—if you run your own server you can set the

It's not really all that standardized. The use of a '+' character to indicate an alias or label is merely convention—if you run your own server you can set the separator to any character you wish, or disable the feature altogether. As far as the RFCs are concerned the '+' character is just part of the account name and there is no reason why it cannot be a mandatory part of the account name on any particular server, such that stripping off the '+' and any trailing characters results in an invalid e-mail address, or even someone else's e-mail account. For sending email or using an email address as an account identifier it's definitely incorrect to treat abc+xyz@example.com and abc@example.com as equivalent. The same goes for account names which differ only in capitalization or placement of periods: some servers are case-insensitive and ignore periods in account names (e.g. Google) but these are server-specific traits and compliant email senders should not assume that every server will work the same way. The '+' alias feature is a fairly common configuration, though, so for source labels it's better to either treat all unlabeled messages as spam or else use a more opaque labeling scheme (unique-hash@example.com) which doesn't hint at an alternative untracked email address.