How do I fake an MX record for a domain?

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http://askubuntu.com – I'm testing two mail servers that live on two (virtual) machines with made up domain names for emails. When one of the servers needs to send an email to the other one, it seemingly performs a DNS lookup for the recipient domain's MX record, which doesn't point the sender server to the correct IP, no matter what IP/domain mapping I put in /etc/hosts. So should I install a DNS server and create a custom MX record for the recipient domain? How do I do this on Ubuntu 12.04 or is there an easier way? (HowTos)