Again it’s an exchange issue,
Welcome back, as the article title assume today issue is that the Transport service repeatedly hangs on Exchange 2013 CU3.
The problem was realized by users complain that they send e-mails to outside and never delivered, and during diagnostics sending from Hotmail to the company mail never received.
Also by reviewing the services the transport service is started and running. but by manually restart the service the mails is delivered correctly to the mailboxes. all that without any error event log in the event viewer.
As fast workaround a script that restart the service every 15 min was created.
After some searching i found the following subject: http://social.technet.microsoft.com/Forums/exchange/en-US/68ddbef1-1565-4a81-835e-444daea37280/installed-cu3-on-a-cu1-exchange-2013-server-and-now-have-to-keep-restarting-transport-service-every?forum=exchangesvrdeploy which describe the same problem, the reason is that CU3 have a bug
that causes the transport service to hangs when you configure custom receive connectors for some applications.
so, to overcome that we are going to downgrade our servers. you also have the option to delete the custom connectors.