You can 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.

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.

So what use are subaddresses? 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-vax mailing list using the address bob+info-vax@example.com, then mail to you from that list will show that subaddress somewhere in the message header. Indeed, whenever you get mail addressed to the subaddress info-vax, then you know that the mail came from that list. You can use this to advantage with mail screening utilities such as the DELIVER system described in Chapter 5.

If you enter the subaddress into your personal alias database as a public alias then you can have mail sent to you with that subaddress automatically forwarded. Continuing with the bob+info-vax example, the PMDF DB commands

db> add info-vax bob@naplesa.example.com db> set info-vax public

will cause mail sent to bob+info-vax@example.com to be forwarded to bob@naples.example.com. The subaddress could have been retained with the command

db> add info-vax bob+info-vax@naples.example.com

The PMDF DB utility is described in Section 9.3.

Finally, subaddresses can serve as simple visual cues to inform you about where a message originated. You can 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.