Gmail allows users to use "+" in their email addresses to create an email alias address of sorts (I say "of sorts" because it's not really an alias set up on the mail server by a server admin or Google Apps administrator [see here]). This little trick in conjunction with Gmail's filtering capabilities can be very useful.
For example, suppose you're working on a team project and you'd like to label all incoming project related messages with a specific label. Of course, you could accomplish this in a number of ways:
- require that all team members put "project x" in the subject line of any project related messages, then filter on subject (but this would also catch non-team members' messages that happen to have "project x" in the subject
- filter based on sender email addresses of your teammates (but this would also catch non-project related messages from these people)
- filter on both sender email and specific text in the subject line
Once this is all set up, whenever you receive an email delivered to the +address alias, Gmail will automatically label it. You can then display only messages with that label, or even set up further actions to do all kinds of other things.