As an anti-spam measure, AOL requires that any mail server connecting to
their mail server provide an PTR record in their DNS server.  That is,
AOL requires the sender's mail server to list which hosts are valid mail
servers at that domain.  You'll need to discuss the problem with
whomever is your e-mail provider because they choose not to include PTR
records in their DNS server for their mail servers, or they might've
recently changed their mail server's hostname but forgot to update their
PTR records, or maybe you are using your own mail server but AOL and
many other e-mail providers will refuse to accept mails from dynamically
IP addressed sources (so you will have to get a static IP address).

http://postmaster.info.aol.com/info/rdns.html

The full text of the error message would help.  But in general that
means that you are using the wrong SMTP mail server (Tools, Accounts,
Mail, Properties, Servers) for your current Internet connection.  Check
with your ISP for the correct settings.  You might need to use SMTP
Authentication.

Which mail server were you trying to use? AOL doesn't accept email from
servers hosted on dynamically assigned IP addresses, among other
restrictions. Your complete error message would have included the name of
the message submission server; which would go a long way toward diagnosing
the problem.

In any case, it isn't something which can be fixed except by using a message
submission server which is acceptable to AOL.