It is actually possible to use symbols. However not for basic AIM customers. AOL sells a software product called Identity services that allows such SNs as “null@null.net”. It’s actually just a lil quirk that they threw in to what identity services is really used for.

P.S. I know this cause I work for AIM :p

Were you an AOL dial-up user recently and happen to have cancelled your account? If so your SN will be blocked from use and you will have to sign-up for a new SN.