Your connection was dropped after the handshake "EHLO", "MAIL TO:" or "RCPT:"
From observation, Aol tends to shift their internal DNS numbers quite a bit, or down a lot of mail servers from time to time. Your own DNS servers may not have cache up in time. This causes your mail to hit the wrong Aol mail server box, and get the 421 code.
There's not much you can do about it. You can try using AOL's DNS numbers for a try. Or someone out there find a way around this.