MMS Failed to Send

I tried to send an MMS to someone who is on the 3 network. It continually fails.
(I'm on NextG). So I sent it to an Optus phone. It worked and arrived OK. I then
did it to a Telstra GSM/3G phone currently roaming on GSM. It also worked. I
then resent it to the 3 phone. Again, it failed.

Why would it fail to send at the NG network end? Doesn't it get into the
"system" then from there it gets sent to whereever it's meant to go?

Whatever, it's frustrating. I shall try again later this morning and see what
happens.