I use HTC messaging to received MMS and use sMMS app to "compose/send" MMS only, Here is how I did it.
Assuming your settings are unaltered:
1. Download the "MMS fix(M2.5)(sprint)(valkyrie) cab. soft-reset
2. Go into your HTC messaging settings/Menu/MMS server/SampleMMSC2/ scroll down to Connect via select "My ISP", Scroll down to WAP and select "None".
This will enable you to receive and download MMS without giving a "Unable to download MMS".
3. Download sMMS app for composing and sending MMS only.
Oh yeah make sure in the HTC messaging settings, Select your desired photo resolution and Camera captured resolution.
"DO NOT" install the Sprint Arcsoft 5.00.XX cab. or the Carrier cab. this will brake the receiving MMS through the HTC messaging app.
This has been tested on My sprint TP2 not sure if it works on other carriers
MMS fix Sprint here
http://thewingster.com/Documents/MMS...(Valkyrie).cab
sMMS can be found a few pages back