Same thing with me. I have a Nokia Lumia 928 and I thought I found APN MMS setting that worked, but then they stopped working. Then I found another setting that seems to work, but when I MMS a message to myself I get "unknown" as the sender and "message not found"??
My current MMS APN settings:
APN: RESELLER
Username: (blank)
Password: (blank)
Authentication type: PAP
WAP Gateway URL: 66.209.11.33
WAP Gateway port: 80
MMSC (URL):
http://mmsc.mobile.att.net/MMSC Port: 80
Maximum MMS Size: (blank)
With that I can send, but not receive????
When I do "receive" it is UNKNOWN & "no message content"