After having used the original Swirly MMS on prior firmwares of the iPhone, I had some considerable difficulty in setting it up once I upgraded both my firmware and Swirly MMS version.
After lots of hassling, it appears that the settings you can obtain automatically from MTN do not work.
The only way in which I was able to get this to work was to change the port number of the Proxy.
I have now tested this for both outgoing and incoming MMS's and it works 100%.
Settings are as follows:
MMSC: http://mms.mtn.co.za/mms/wapenc
Proxy: 196.11.240.241:8080
APN: myMTN
UserName: mtnmms
Password: mtnmms
If I am not mistaken, when requesting settings from MTN, they give you a port of 9201 for the Proxy - this does not work. It needs to be changed to 8080.
Hope this helps!!