Since I found this solution by chance, even after searching, I thought I would post on here as it might help others one day.
A new client came on board and when their DNS updated to point to my hMailServer install, they were getting the message "connection to the server was interrupted" from Outlook. They were running AVG etc so I thought it was that, once I had eliminated that I started looking at other things in hMailServer, such as locked mailboxes etc. I knew it wasn't a routing issue as the logon times for the mailboxes were still being updated.
So, eventually I found it - the reason I couldn't put them onto hMailServer ages ago because of an apparent connection problem and why saslauthd on the other mail server use to consume vast amounts of RAM - they had a fax-to-email server constantly checking a non-existent mailbox! So what did hMailServer do? Added their IP to the Auto Ban list!
I could have found that a lot quicker if I had switched on pop logging perhaps, but as all other clients were running, and I could receive their mail from my PC, I assumed it was a problem their end.
I hope this helps someone else who like me, didn't check the now obvious.