GreenAnt email allows the use of "plus addresses".
This is a useful feature for limiting spam to your address (or at least knowing where it comes from).
Let's say we have an email address:
Now Bill wants to sign up for a newsletter at
Fish Ice Cream.
He'd like to have some idea if emails come from that address and be able to apply mail filers.
When he signs up, Bill can use the email address=:
The emails will come through as usual but now Bill has some ability to know the origin of the email.
This can also be useful for automatically sorting emails by sub-category or advanced filtering.