"Last week AOL announced on its postmaster blog changes to the way it will be handling mailer daemon errors. 

What does this mean for large-volume email senders? You should expect to see a change in the From: address, as well as the number of asynchronous bounces you receive from AOL. Asynchronous bounces occur after the SMTP conversation, which means that the ISP accepts the senders' email first and then rejects it later. As a result, the bounce notifications trickle in minutes to days after the initial send in the form of an email. This is different from synchronous bounces, which occur during the SMTP conversation. Most MTAs record those bounces in the form of a log entry."

Continue reading here: Changes to AOL Bounce Processing :: Return Path Blog.