Two points. First, what you are describing is, from the point
of view of AOL, not really spam. While I agree that unsolicited
commercial email is spam, normally the problem is sending a huge
amount of identical messages within a short time. You aren't doing
that (unless a lot of your users are all getting the same spam message
at the same time.)

Second, and more important here, it should be obvious from the
headers what is going on and AOL shouldn't be penalising you.
I normally receive my email at home but, if there are problems,
someone trying to send me email will end up sending it to one of
the backup MX servers, which will then later forward it on to me.
The last link in the chain is then this backup MX server, but I would
be wrong to complain that it is sending me spam.

I don't really see the point of your customers having an email address
with you if they just forward the stuff to their AOL account. Why
don't they just have stuff sent to the AOL account?