CMN Mail Connector fails with "A communications failure or interuption was encountered" (4288577)

The Coexistence Manager for Notes (CMN) Mail Connector log shows the following warning:

2013-05-21 11:11:11,906 WARN  CMN.logging [Proxy 105] - Message Number [105] on Thread [104] from Domino (10.10.10.10:3064) to Exchange (10.10.10.11:25): A communications failure or interuption was encountered. - IOException: Unable to read data from the transport connection: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.
Stack Trace: at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at QuestSoftware.CMX.API.Core.Networking.SecureSocket.Read(Byte[] buffer, Int32 offset, Int32 count)
   at QuestSoftware.CMX.API.Core.Networking.SecureSocket.Read(Byte[] buffer)
   at QuestSoftware.CMX.API.Core.Networking.Smtp.ServerProtocol.ReceiveMessageBody(TempStream stream)
   at QuestSoftware.CMX.API.Core.Networking.Smtp.ServerProtocol.ReceiveMessageBody()
   at QuestSoftware.CMX.API.Core.Networking.Smtp.Proxy.ReceiveMessageBodyAndProcess(ServerProtocol server, Boolean messageIsDsn)
   at QuestSoftware.CMX.API.Core.Networking.Smtp.Proxy.Run()
   at QuestSoftware.CMX.API.Core.Networking.Smtp.Proxy.<Start>b__0()
SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
Stack Trace: at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)

• To confirm the issue, telnet into the same IP:port mentioned in the warning
• Adjust the firewall rules
• Check if your firewall has network inspection enabled for SMTP and ESMTP traffic and the limits set.