There isn't really enough information to answer your question directly, but maybe there are some pointers.
First off, I assume you have an AOL account you're trying to use?
You also have other accounts (e.g. gmail) setup on your system?
Like most email providers, AOL (and gmail) implement filters to determine who can send mail through their system.
In AOL's case, you can send mail through their system if you're an AOL customer using an AOL account/email address, or if you're sending mail to an AOL user.
What you can't do is send mail to AOL if neither the sender nor the recipient are AOL users. To allow that would open AOL to a slew of spam bots leveraging their system to send mail.
So, conceptually:
user@aol.com -> user@somewhereelse.com -> OK - sender is valid AOL user
user@gmail.com -> user@aol.com -> OK - recipient is a valid AOL user
user@gmail.com -> user@somewherelese.com -> Go Away! - neither sender nor recipient are related to AOL.
The reason I say this is the only reason I can think of that error message (other than AOL's servers being offline) is that you're trying to send a message from your Gmail account to some other user, through the AOL server.
Your Gmail account MUST be set to send outgoing mail through Gmail's servers. If you try to send a message from your Gmail account through AOL (and it's not addressed to an AOL user), then AOL will reject the message.
Check your mail client's settings. Assuming you're using Apple Mail, that's in Mail -> Preferences -> Accounts.
For each account (AOL, Gmail), select the Server Settings tab and make sure it's set to the appropriate Outgoing Mail Account.
If that looks correct, then post back and we can dig deeper, but off-hand that's the most likely cause I can think of.