IMHO this is (sometimes) normal behavior. Your mailserver should try to deliver a mail three or four days.
Greylisting for example refuses the first delivery if the triplet (sender, IP, recipient) is not known.
If a mail-relay is down for a few hours, your mailserver has to try again.
AOL sometimes is a problem. It does not tell you: "Not found" and sends the MAILOR DAEMON back to you ... no, my mail-relay says:
After five days: "schnuff.di.wuff@aol.com ... finally given up", which means: "This recipient does not seem to exist."
There is nothing you can do about it. On the other hand ... you could reduce the number of attempts (max. 2 days), but this is not recommended, because a mail-relay could be unreachable (e.g. weekend).
Regards,
Claus