Encrypted traffic dropped?

[barcajax () gmail com]

I have recently installed SimpLite-MSN software (http://www.secway.fr/us/products/simplite_msn/) to encrypt my IM 
messages. For this software to work, it requires both the sender and receipient to both have this software installed 
and running during the IM session. SimpLite behaves as a proxy so MSN Messenger talks to SimpLite locally and SimpLite 
will exchange traffics with the MSN network. How this works is that public keys of both parties are exchanged for 
authentication purposes and the IM messages are encrypted with symmetric AES 128 bit encryption.
I convinced friends to start using this software to encrypt all our chats. Only one friend (located in UAE) has 
problems when using SimpLite. We are able to exchange public keys but subsequently after that, that friend is not able 
to send or receive IM messages. Seems that his ISP is able to recognise that the packets that are being exchanged are 
encrypted and dropped accordingly because he is able to resume using MSN Messenger only after turning off SimpLite.
Is my hypothesis correct? If yes, would anyone hazard a guess how the ISP is doing so?