Steve, since you are using multiple IPs on your system, you need to identify
to the email program which IP it is using. I can be on the same IP as the
Web Server, it really isn't a problem but you need to tell the program you
use for email service which IP to listen on.
It sounds like you have mixed up a few things (welcome to the club!) Make
your mail server accept mail for yourdomain.com. Setup the MX record to
point to the name of the machine you have configured to use in the mail's
setup. Make an A record for yourdomain.com and put it in the DNS server (it
doesn't matter if you are using that for other things as well - note: this
is where others may disagree with me but it has worked for me for over a
year). You should be up and going.
Do not use a CNAME record for a mail server. If anything, use one for your
www.yourdomain.com instead. Many other mail servers expect to do a reverse
lookup of your IP address and it should return the name of the machine used
for the MX record in the DNS. If this is not done, that will screw things