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
email@example.com is a subaddress.
Subaddresses do not in any way hinder your mail from being delivered to you. If your address is firstname.lastname@example.org, then messages sent to email@example.com will be delivered to you just as messages to firstname.lastname@example.org 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 email@example.com. Now, if you subscribe to the info-vax mailing list using the address firstname.lastname@example.org, 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
db> add info-vax email@example.com db> set info-vax public
db> add info-vax firstname.lastname@example.org
DButility 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
To: address shows.