Let's see if I can explain the situation clearly.
I have a WHM/Reseller account.
Under that Reseller Account I have two domains. One is my main business website (say, mywebsite.com) and I have a customer website (say, customer.com).
The customer has an inhouse MS Exchange server with its own dedicated IP address. So, in WHM, I create MX records to point at:
and I create A records for smtp.customer.com and mail2.customer.com which both point at the same dedicated IP address.
This works fine for the customer's inhouse users and just about anyone else on the planet sending email to the customer.
But I have problems, presumably because my mail domain is on the same server as the customers' website. If I use my mywebsite.com smtp server to send an email to, say, firstname.lastname@example.org it does not get delivered to their inhouse MS Exchange server, instead it is treated as unrouted mail on our common web server and ends up in the customer.com catchall account (or :blackhole: ) depending on how I set up the defaul email.
If anyone has a suggestion as to how I can get my outgoing email to get out of our webserver and delivered to the customers' inhouse mail server, it would be greatly appreciated.
[One idea, of course, is to use a smtp server other than the one on our common webserver... Unfortunately, my ISP's smtp server won't allow outgoing emails with a return address other than my ISP mail account.]