I found this about AOL's mail policy: http://postmaster.aol.com/trouble/mailer.html
550 Requested action not taken: DNS FAILURE
These errors indicate that there are DNS resolution problems somewhere between your domain and AOL. This usually means that AOL is unable to resolve your domain name in DNS before accepting that mail for delivery. Contact your e-mail administrator for assistance. The network administrator for the site may have to modify their resource records in DNS.
So there seems to be something wrong with the DNS records of the sender domain. What's the sender domain you use?