Thank you for your reply.
Yes we are on a NAT'd LAN.
Client DNS is solely the local server
Server DNS service only provides our one local domain, has forwarder adresses set to 18.104.22.168 and 22.214.171.124 (Google).
I am not confident, but I don't *think* this is the problem because DNS takes some time to timeout (15s?) and the error message above comes up almost instamntaneously. ie too quick to be a DNS timeout.
If it was a DNS problem, what would I see in which log? named.log on the server is empty.
I'm not sure what to look for in logs on the client?
Thank for any further ideas.
To verify the server DNS, launch Terminal.app from Applications > Utilities on the OS X Server box and issue the non-destructive diagnostic command:
sudo changeip -checkhostname
The clients would have references only to your local DNS server(s), visible via the client Network Preferences settings. (These DNS server settings are usually either hard-set in the client, or acquired from DHCP by the client.)
You'll want to use Console.app or direct access to the /var/log directory (on the server), and see what gets logged when the mail message move operation fails.
As a preemptive operation, I'd also rebuild the mailboxes involved (on the client(s) involved). I've seen a few mailbox corruptions over the years, but not one that caused this behavior.