Help unknown network error message when i try to send mms

[Mr. Boom]

Twice now i have an error when trying to send an mms. I get "unknown network error" and "generic network error" messages when it fails. I am unsure what caused it either time. First time i did a battery pull, and it started working again. Today, i just put it into airplane mode, then back, and it worked again. I saw on another thread something about APN. What is APN, and how do i check it? And what could be causing this?

I never lost cell signal, or 3G at all when it happens.