I was unable to determine the reason mail.eskimo.com was returning an error code when trying to send. There was no resource exhaustion I could identify, plenty of RAM, Disk, load was low, memory not exhausted. Restarting postfix didn’t fix it and there didn’t appear anything bad in the queue.
I opted for the Windows solution at this point and rebooted the server. After the reboot it accepted and sent mail again.
A post mortem analysis of the logs showed that the smtp daemon was unable to open /dev/null. Not sure why it needs that but apparently something did go wrong in the operating system kernel. This is the first time I’ve ever seen a kernel issue on that version of Linux.