Pobox supports the use of "+anystring" (plus extensions) with any address. We forward and process email@example.com (or any of our other domains) the same way as we treat firstname.lastname@example.org. This lets you use the value of anystring to filter your mail.
For this reason, we do not permit people to select addresses for their accounts that include a "+" in the username. For example, if we let you select the address "email@example.com", then the person using "firstname.lastname@example.org" could not use "+" extensions without colliding with your account.
If you are reading this, and have no idea what it means, some people use + extensions to track the usage of their email addresses, and block spam. For instance, you might sign up to receive emails from YoYoDyne, Inc., and give them the email address email@example.com. That way, if they ever sell your address, you know, and if they suddenly start receiving spam at that address, you can just throw away mail sent to myaddress+yoyo, instead of having to change your Pobox address. (If you have a personal domain, people also use AllMail for the same purpose.)