Another thing to keep in mind is that Sailfish MMS is unable to do DNS resolution over MMS data connection. It’s doable, it just wasn’t done and it’s fair to assume that it’s never going to be done, since MMS is a dying technology. Which means that in order for MMS to work reliably (without another data connection being active at the same time and providing DNS resolution) the MMS proxy should be there and it should be an IP address.