Thanks for the suggestions Olaf.

 We were not using SSL for our IMAP connections to the server and they are on port 143.  I will investigate the SPA settings in Outlook to see what they are.  

I did discover that outlook appears to default to checking for emails only once ever half hour. 

After a rather silly error which resulted in me deleting most of the data on our mail server two of our users who have Outlook as their mail client do not appear to be getting any new email.  One of them also has Pegasus set up and reports that the email is working fine on Pegasus, the other has several other IMAP clients set up on his android devices and again reports that all of them are OK.  Has anyone had similar problems with outlook apparently not synchronising their emails when other clients were OK and if so did you find a solution?

Thanks

Chris 

What data did you delete? May be configuration data and/or certificate?

May be you enabled other ports or encryption (SSL/TLS) than before?

What protocol do they use: POP or IMAP? Is their an IMAP-/POP-Connection or is it refused (have a look at Mercurys protocols).

Outlook works here with SSL on Port 993 for IMAP and TLS on port  587 for SMTP.

Maybe they have enabled SPA for authentification in Outlook?

bye   Olaf