Peter wrote:
But they get accepted by AOL? What do your server logs say?

That sounds like greylisting.

Yes, it can be a problem as the others have said. Checking blacklists
and server logs on a daily basis should help though, and if you are on a
blacklist the provider usually offers information on how to get off
their list again.

Regards,
Gunde

Peter wrote:
I can think of several ways:

a) The server gets compromised even though you thought it is secure

b) A user sends mails on purpose that are UCE/UBE

c) A machine in the customer network gets compromised and uses
the company mail server for sending out UCE/UBE

d) Somebody classifies the mail as UCE/UBE and reports
it to one or several blacklists

e) Somebody decides to put the server or the network it is connected
to on a blacklist due to reasons known to himself, and it does not
always mean that server has done any spamming:
http://www.spamhaus.org/sbl/sbl.lasso?query=SBL55483

There are probably more scenarios out of which the mail server owner
cannot influence all directly.

Regards,
Gunde