I would contact AOL and ask them. Sometimes, their anti-spam mechanisms can be too enthusiastic and the fact you are bulk-sending may be triggering that mechanism. If you contact them you can ask them to whitelist your sending address.

Strange problem:   Have version 4.72 dated 2/19/16.   and Windows 7.    Am getting permanent error on all AOL.COM address in my distribution list.  But if I send individual mail to same AOL.COM address it goes without a problem.   Is the problem with Pegasus or with AOL?     Distribution list is only 25 address long and recipients have made sure that their systems will accept my email.  Any ideas?     This problem just started about three months ago.     Thanks,  Al