How to Play MP4 in Sony Ericsson Mix Walkman phone

Hi,

I forgot to ask one more question, I am unable to play mp4 in this phone , I tried but can play few but not all, what is the solution. I think I have bought wrong phone. Please help to fix this problem.
Just to tell , my phone is Sony ericsson mix walkman WT13i purchased few months back.

Yes, it can play MP4 but if you have high resolution MP4 then you can't play , you need to reduce its resolution . I think it supports only QVGA mp4 playback.