D'Oh!

Good thought Brian, but no that wasn't it ... Aderoy hit the nail on the head. 

 Coincidence struck!  After being away, a lot of mail I hadn't sorted out yet was still in the new mail box and it tipped over the magic number the very day that the system decided to release its factory installed smoke!

PMail 4.63 (Win32) build 325  POP3/SMTP config   Auto check mail every 5 mins from primary mail server

 Windows 7 Pro (64 bit)

 Intel i5-2500K Ivy Bridge Chipset Mother board

I had a system disaster that required me to change the Mobo (and PSU)  Previous mobo was Sandy Bridge based ... same processor

After the upgrade, I had to do a lot of work replacing drivers etc. and the system still doesn't shut down properly but it seems to run OK, except for this PMail problem.

When I get new mail, the notifier window pops up, but the new mail child window does not display the new mail entries, as it used to.  If I close and reopen the newmail window, the new mail entries now appear.

The notifier window no longer alternates between total new mail and new mail since last check when clicked.

Any ideas?

Check the setting that controls the new mail folder view refresh timing.  It is located in Tools | Options | General settings | Advanced settings

Also if there are more than 750 messages in the New Mail folder it will not update properly iirc.