AOL is currently experiencing issues which are causing incorrect error codes to be issued. This is an issue on AOL's side and they are currently working to resolve the issue as quickly as possible. We will keep you updated as soon as they have a resolution. Here is what the AOL Postmaster Blog is reporting...
"We are at present issuing a "451 4.3.0
This is not replacing the existing two codes:
500, text= 5.1.1 : Recipient address rejected: aol.com
and
550 Mailbox not found
Senders using scripts to remove unknown users automatically should be looking for all three codes for the moment.
AOL AntiSpam"