AOL hasn't yet mentioned this on their Postmaster Blog, but multiple sources are reporting that AOL's MX records temporarily vanished overnight. It's not clear how long the issue lasted, but based on the TTL (time to live) cache set for the AOL domain, the error persisted for at least an hour. I saw one ESP notice the issue about 1:15 am central time.
While the issue was ongoing, your attempts to email AOL users would result in a hard bounce. No MX or A record was found in DNS for aol.com during that time.
The issue seems to be resolved as of this writing (10:45 am central).