Why can I not send MSN emails when on Mobile Network with my HTC Inspire 4G

When hooked up to WIFI I can send and receive MSN email, but when I switch to Mobil Network I can receive MSN email but not send.  I either get an error that the system could not authenticate my login and to check id and password, which I have done many times over, or it says the settings are wrong and I have double checked these as well.  The last ATT support person said it happening do to some app I have on my phone?  GMAIL works just fine.