Jonathan Mendez notified me of a Google Groups Thread that shows many reporting that their Gmail emails sent to AOL are being returned. The error code people are receiving back from AOL is Error RLY:CS4, which is defined as;
EXPLANATION: This error message indicates that your email has been identified as coming from a server or web site that may have a compromised script or program available.
SOLUTION: Please contact the server administrator or hosting provider for additional assistance.
Providers: This error message will allow the offending emails to stay in your mail queue. This will give you the opportunity to look at the offending mail, which should assist you in tracking down the compromised script.
No reports back yet that this is being worked on by Google or AOL.
Forum discussion at Google Groups.