Same issue here. We have MMS notifications from geocaching set up so we both receive them, we are both on AT&T iPhones, though we have different models. We are getting getting other mail-to-MMS notifications, but ones from geocaching are not getting through. Seems like this happened to us in the past and we were told to check with our carrier but got nowhere with calls to AT&T. The last texts we can confirm were 4/19/2020. We have tried playing with the last text we received and can send them between the two of us using the mail to mms route (xxxxxxxxx@mms.att.net address) if we take out the “http://coord.info/GCxxxxxx”. We can also directly text that url. However the AT&T mail-to-MMS gateway seems to be blocking a message that contains a url of that type. We can also send other urls through the mail-to-MMS gateway, so it seems to be the just the coord.info urls that are getting blocked. My guess is something in the AT&T mail-to MMS gateway is blocking or failing to handle those urls. Hopefully someone is able to get to the right support people in AT&T to gat this fixed.
