There are several reasons to use “plus addressing,” or “subaddressing,” as it makes filtering and acting on emails much easier. Email addresses can be created for the handling of mailing list traffic and other specific online communications.
In practice, plus addressing does make filtering easier. However, using plus addresses to sign-up for online services does have the drawback that the email address must be typed in exactly as it was registered.
The function forgot my password will rely on the email address exactly as it was registered with the website or mailing list. The email address provided must include the “+” part of the address if it was included during registration. If the “+” section of the address is forgotten or otherwise misremembered, the email address will not be recognized and it will be harder to make changes to your online account or subscription.
For example: If you owned the email address johndoe@example.org and wanted to register for a pareidolia mailing list. The email address supplied to the mailing list server might be something similar to johndoe+pareidolialist@example.org.
The use of plus-addressing typically requires maintaining a copy of the registration email that can be searched inside your email inbox. The reason for keeping a copy is because we inevitably forget exactly which email address was supplied to which services, because all email just appears in our regular inbox.
Can I reject emails that do not match any “+” addresses?
It is a common question where we try to set up a clean mailbox by rejecting every message that does not include a valid “+” address. There are issues with this particular approach as it does not abide by SMTP governance. It is NOT possible to do within the SMTP protocol.
The best course of action is to have a final filter (after all other “+” filters have executed) that sends a polite message back to the sender. The message could say something similar to:
“My email address uses appropriate + address filtering, which means in order to get a message to me, it must match an address I’ve set up. Emails sent to myemail@example.org without any + signs are automatically discarded. Please contact me at myemail+nofilter@example.org to get a message to me.”
This is an important factor to recognize because many of us have tried to this particular approach without any success. The reason it does not work is because the receiving MTA (Mail Transport Agent) sees the email address as valid, thus immediately returns a “valid” or “delivered” code to the sending mail server. (One caveat is the valid code is only returned if the message is not denied for another reason, like insufficient quota or user as been disabled.)
Once the email has been accepted, it is passed on to the MDA (Mail Delivery Agent) for processing and delivery to the correct email box. Sometimes the MDA is a program, like procmail or spamassassin, which are email filters themselves. There are also other mail processors which can read messages, understand headers and do other things. (Sometimes these programs handle email addresses for organizations to avoid shared inboxes.)
Hopefully this will be of some help and avoid many hours, days, or weeks of headaches trying to do something that is not possible. (e.g. deny emails that do not match an existing + filter.)