Really stuck with this Outlook problem

I purchased 4 brand new Optiplex 755 computers. All of them are experiancing the smae issue. Dell is not helping because it is a Microsoft problem, not theirs. If any of the users open Outlook, add an attachment and do not send for a few minute, then click send the Outlook application crashes and says ‘network connection lost’. The network connection is there, they can send other emails while this one is open, they can surf the Internet etc. I have looked everywhere and cannot find the cause of this problem.