DNS MX and AOL

I run sendmail on my firewall (for the past 3 years) and have been
having trouble receiving emails from AOL users for about two weeks. My
ISP has two MX records for my domain, one for my firewall with priority
10, and one for their mail server as a backup with priority 100. The
one thing that I seem to recall is configured incorrectly is the MX
record for my sendmail server points to a DNS CNAME record rather than
an A record.

The failure symptoms are intermittent bounced email to AOL users
attempting to send email to my domain with an error message indicating
that the AOL mail delivery agent and my ISP's mail server session timed
out. I haven't had any problems for other than AOL users and this has
only been happening for about the past two weeks.

Is it possible that AOL is attempting to contact my ISP's mail server
rather than my email server because of this CNAME vs A record
configuration on my ISP's DNS server?

Any ideas?

Thanks,

-Doug WInterburn