Picture messaging. I have a Sony Ericsson K550i and I have not been able to send or recieve

Question

I have a Sony Ericsson K550i and I have not been able to send or recieve picture messages I have been into The sony Ericsson web site to recieve the settings and my phone recieved them successfully , but still will not let me send or recieve I just get the envelope symbol and the globe flashing at the top of the screen for ages and t never gets sent. Please can you offer any suggestions ?cant send mms but can recieve