Outlook to Exchange Disconnection

Hi There,

We have 4 sites (A,B,C & D) ,all connected to site A with an MPLS connection.Exchange server resides on site A and clients from all sites access emails through Outlook.Request your guidance on issues we come across;

  1. Some of the clients keep getting an authentication popup (Even on Site-A).Restarting outlook may solve the issue.

  2. A particular user on Site-B repeatedly complaints about  a non response from outlook and when we try OWA ,It says "Mailbox           Un-available ".But it re-instate to normal after a while .

Request your advice on both matters.

Thanks In Advance,

Ratheesh Nambiar