I'm presuming that you are not on the AOL domain when you attempt to
send an e-mail to an AOL recipient. The Received headers in the NDR
(non-delivery) e-mail would show from whom the NDR was sent: from the
sender's (your) own SMTP mail host or from the recipient's SMTP mail
host. I can't tell if it is your SMTP mail host that is rejecting your
e-mail or if it AOL's SMTP mail host (after your SMTP mail connects to
it) that is rejecting your e-mail.
Have you tried sending a simple plain-text (no HTML) test e-mail through
your SMTP mail host to another domain, like to a freebie account
elsewhere (and not AOL)? If that works, try sending a plain-text test
e-mail to your AOL recipient.
When you send e-mail, are you on the domain for whose SMTP mail host you
are using? That is, is the SMTP mail host you use on-domain to your
network? If not, you'll have to authenticate to that off-domain SMTP
mail host. Why? Because it doesn't know that you have permissions to
use their resources. That is, they can't tell when you come from
off-domain if you are their customer and are allowed to use their
resources. Go into the e-mail account defined within Outlook and enable
authentication on the outbound (SMTP) mail host. Although you can
select to reuse your POP login credentials, I always enter my login
credentials in that options dialog window.