How to fix a Postfix/MySQL/Dovecot Unknown Host Issue?

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http://serverfault.com – I am having an issue with one of my Postfix/Dovecot mail servers and I'm unsure how to fix the problem. I will try to explain it in detail, here it goes: I have an Ubuntu server setup using Virtual hosting with Postfix, Dovecot and MySQL. We have one domain setup as a virtual domain, for this example I am going to use mail.example.com. Under that domain we have one email address. I have another server (MS Exchange) setup using another one of my sub-domains, ex.example.com. The problem is that when I SMTP into the account on mail.example.com and try to send an email to an account on ex.exampl (HowTos)