Out of interest, I was wondering about the URI used to reach the MMSC
if the host component is a name, it would need to be resolved.

if multiple connections are made to different APN (or even wifi + MMS APN) split DNS would need to be taken into account. To me it would seem reasonable that the host name of the MMSC may only be resolvable when using the DNS server presented when connecting to the MMS APN.

I guess that rules out using the system resolver. I guess but it it complicates the idea of using NAT or some other IP translation and routing manipulation since multiple DNS server entries may also need to be taken into account. And the HTTP client would need to rely on custom DNS code.