#5.2.1 smtp; 521 5.2.1 : AOL will not accept delivery of this message
So after checking all of the obvious such as if any of the outbound servers were on any RBL, they weren’t. Email wouldn’t send but would send from a different server, then I removed the signature and the email sent!
I can only assume this was down to links to the images in the signature that AOL took expectation to. 🙂