Any ideas on portal email notification issue?

We have a production instance of PD 16.5 SP3 Portal running and we have had email notifications working fine for several months, but after the holiday period it seems to have broken :-(.

When ever from the "Repository Explorer" we select the "Subscriptions" tab the status icon moves into a "Disconnected state"- .

When looking at "Settings\Notification" that all looks good - and the "Test Mail Server" looks good. Additionally in "Settings\Server Status" the "Change Notification Information" is running.

We have bounced the portal, and looked at both the Server & Client log files (with debug logging on) - and see nothing that stands out. Additionally we have mail working on the same box using different applications (this helps show its not a network firewall/machine permissions), and on a different portal pointing to a different repository.

Has anyone seen anything like this, know the solution or a way to move diagnosis forward? Any ideas on what the Disconnected state means in this scenario?

Many thanks,

Gareth