I just set up a new Courier-MTA server and came across this issue while testing. A message to an external email address will be delivered, but there is a five minute delay between the server receiving the message and delivering it to the destination mail server. I have compared config files between this server and my operational Courier server and can find no differences that would cause this. I'm including a clip from my log file below:
Sounds like a DNS lookup problem. Check your /etc/resolv.conf and make sure it's pointing to functioning nameservers.
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