You may include extra information in your mail address. This extra information is referred to as "subaddress". Any material following a plus, +, in your address is considered to be a subaddress. For instance, "test" in the address bob+test@example.com is a subaddress.

For BSD mailbox users, subaddresses do not in any way hinder your mail from being delivered to you. If your address is bob@example.com, then messages sent to bob+test@example.com will be delivered to you just as messages to bob@example.com would be.

For PMDF MessageStore users, subaddresses control which folder your messages are delivered to. If your address is bob@example.com, then messages sent to bob+test@example.com will be delivered to the folder named "test" of your bob MessageStore account.

Subaddresses are useful even for BSD mailbox users for whom they are merely "cosmetic" information. One common use of subaddresses is to track where mail is coming from. Let us again assume that your mail address is bob@example.com. Now, if you subscribe to the info-unix mailing list using the address bob+info-unix@example.com, then mail to you from that list should show that subaddress somewhere in the message header. Indeed, whenever you get mail addressed to the subaddress info-unix, then you know that the mail came from that list.

Finally, subaddresses can serve as simple visual cues to inform you about where a message originated. You may elect to always include a subaddress in your address when you tell it to other people. It is then possible to figure out where a message came from just by what subaddress your To: address shows.