email address with a "+" char illegal? why?

I tried to change my email address to something like myname+fromtoms@gmail.com to make it easier to filter the incoming messages. The change was rejected with the error message "Updated email address appears to be incorrect!".

What's the problem? Is it the "+" character in the email address?

If you're curious what I'm trying to, please look at this short gmail help page. It describes how to "personalize" the email addresses you use with gmail. How to receive messages at a (slightly) different Gmail address

-john