That's lucky for you. I have no problems with anyone besides sprint. Maybe its that certain phone, or the problem only occurs on some HTC Aria's.
My one friend who does have it will just have to upload to an FTP site using AndFTP.
It's not just the Aria, there are many causes for this, but we are unsure how to fix it. I run into this at the store on occasion. First things you want to make sure is that the person on the other network hasn't accidentally added you to their block list, in regards to texting, because some phones have that capability.
Second, you need to make sure that there isnt a data block on you line. This should be taken care of since you have a data plan but it has been known to happen even though you have a data plan the data block doesn't automatically fall off.
If there is no data block on your line, thats good, but there could be a data block feature in the family group features as well and that will also keep you from getting mms messages. This feature is usually added when someone on the family plan has decided to block all data to all phones at the same time for various reasons, but they often forget that they did that and blame us for it
Try changing the SIM card as well... been known to fix issues like this as well.
Those are the things I have seen in the store, and have worked in the past... other then that, there have been a few cases where we cannot find a fix. Hopefully yours is not one of them.