Get them to fix their broken systems? The concept of a "label" exists only in the user agent. To everyone else the '+' character is just part of the account identifier, and a perfectly valid element of any e-mail address. The entire portion before the '@' sign is potentially significant. If someone hands you an address abc+def@example.com, there is no guarantee whatsoever that mail sent to abc@example.com will end up in the same individual's inbox—that could be a completely unrelated account.

On the e-mail provider's side, one way to work around such broken systems would be to let users generate random, opaque aliases which do not contain their primary e-mail username but route to the same account, with a predetermined label applied on receipt. These would look just like ordinary addresses and there would be no way for services to strip out the identifying elements.