I'm on T-Mobile in the US and have found that none of my sailfish phones (tested Pro1, Xperia X and OnePlus X) are able to download MMS messages when connected to wifi but can just fine when on cellular data. Obviously I don't want to use data for everything - but any ideas of how I could force it to use the cellular connection when downloading an MMS?

So as long as you're not at home/work/regular hotspot and therefore not connected to wifi, the MMS downloads as expected, right? That's weird, indeed. In my experience, with both Xperia X and OPX, the phone always used the MMS associated data connection to get the MMS data. And if I didn't have the wireless data for MMS set up correctly, it wouldn't work at all, wifi or not. I'd ask if the mobile data settings were right, but since you get the MMS properly on the road, that's not it . . .

I'm on an AT&T MVNO in the US, but I haven't had that particular problem.. I don't have any other ideas, but if you need anything tested that I can help with, l'm happy to do it.

yeah, I assume it's something T-Mobile specific. It definitely seems to be about the wifi connection, because MMS also works when wifi is enabled but not connected to anything.

You did not mention whether it happens on any WiFi or just one. You are most probably right and it is something operator specific, but there is still a small chance that your router or ISP filters that port or protocol.

(Caveat: I may be talking utter tosh. I may have used MMS maybe 3 times in my entire life.)

It happens on my home network (comcast) and my parents' network (verizon) so unless both are filtering it I doubt it.

(Also when I boot my Nexus 5 into Android with the same SIM, it can download MMS even when connected to wifi.)

Sailfish MMS on T-Mobile US has been a long-running issue:

https://talk.maemo.org/showthread.php?t=99925

It's generally just an annoyance, but my experience is that MMSs become inaccessible if they're not downloaded in time. I've missed a few messages that way.

I've learned to live with it (we have bigger issues coming with the eventual US carrier switchovers to VoLTE). But yes, it would be nice if there were a solution that didn't involve manually turning off wifi when wanting to view an MMS.

taixzo, have you had any luck with the other suggestions? I just remembered about some problems with some carriers and mms when the message center was a URL instead of the numeric IP. Something about proxy not doing DNS lookup right when not going thru mobile data connection.

Just a random thought, better type it before it's gone.