Some senders, most of them hosting companies, have been having issues mailing to AOL. This started last week and is still occurring. A botnet which had infected some machines began sending mass amounts of spam to AOL. We are actively working with hosting companies to help them address this issue. We have seen some improvement but the issue is ongoing.
If you have a hosted domain, please contact your hosting company for next steps. They may have instructions for you involving their servers. If you are a hosting company, please get in touch with us via our Postmaster site.