we are having problems sending messages from the support desk since moving to exchange 2013

logs from the exchange server imply that the support desk is trying to connect using a different port each time in spite of being told to connect using port 25. ports used in my recent trials include 53286, 53293, 53305, 53348, 53364, 53366

2015-05-05T09:57:36.293Z,SERV-EXCHANGE\support desk,08D2249FD6705866,2,10.49.4.46:25,10.49.4.37:53286,>,"220 SERV-EXCHANGE.domain.local Microsoft ESMTP MAIL Service ready at Tue, 5 May 2015 10:57:35 +0100",
2015-05-05T09:57:36.294Z,SERV-EXCHANGE\support desk,08D2249FD6705866,3,10.49.4.46:25,10.49.4.37:53286,-,,Remote(SocketError)

our exchange server is 4.46 and the support desk is 4.37

can anyone advise?

Thanks,

Derek.