Spot Protect has migrated mail servers to a new host. The up side is that they have increased their deliverability from around 96% to 99.9% (less servers are sending notifications to spam). They now configure the “From” address of the email but there is a secondary header that is a “ReplyTo” header, and it is a generated address that the server prepends to the registered root sending domain (sensornotice.com). Most carriers seem to use the “From” address so the SMS’s are always from the same place. AT&T uses the “ReplyTo” address which changes with each email.
They’re also trying to keep an eye out for other options, such as having the server change how they generate their messages, but they are almost 100% focused on deliverability to inboxes (which is a good thing), so if they deem that this gives them even a slight advantage there, it’s unlikely that they will change how they have it set up.
So at the moment there are really only two options to get around this issue:
Change cell carriers (again this seems to be almost exclusively an AT&T issue).
Call AT&T and ask them to change how their SMTP to SMS service works <-- With enough people complaining you never know. I know this isn't ideal, but it seems like at the moment the problem is unavoidable with AT&T, and in the big picture has provided a more reliable service.