Look at the network not area (T-Mobile), it is very likely they are served by the same MMSc if same network.
If the MMS had content when submitted to the MMSc (i.e sender didn't send blank) then the problem may relate to content adaption at the MMSc, possibly only from some originating handset/file types. Content adaption may be having problems converting the file to something it believes is compatible with the BB MMS client/display capabilities.
If you get a blank MMS from a user on the same network (easier for them to look at) and this is blank, then note originating handset type, find out size and the file types which were included (contact originator) then report it to your network.
Possible causes of blank MMS are -
Content adaption cannot convert file for display at receiving device*
Content Adaption/AntiVirus has removed an S60 MMS propogated virus or similar*
The content or header of the MMS has become corrupted, originating carrier -> delivering carrier
*These should result in a fallback route, either advising the MMS client that it can't handle the MMS or sending of www link to the MMS via an SMS to recipient. Some networks won't support this however.
Content adaption has a whole lot of variables to consider, some handsets/software versions don't even originate files correctly formatted for the type, receiving the blank rather than knowing why the MMS cannot be displayed may be the only error. As above, I'd let them know however.