How can I get text messages sent via email to appear from a single sender rather than each one being unique?
Best answer by srickar
Those three slashes signify the plain text ASCII message format sender / subject / body. Why? Plain text SMS is the sum of the sender + subject + body = (<160 char). Plain text is the lowest denominator decision. If less than 160 characters, RFC standards commit the message to traditional SMS delivery because there is no transcoding or HTTP get required for such a basic message format.
When it's redirected to SMS, the sender header info is dropped in favor of SMSC gateway identity. The workaround is to force the messages as MMS which would be HTML, multimedia, longer than 160 characters, or include a subject which should force it to remain as MMSC message.
I understand this is a pain, as firefighters use them for CAD paging. This design went into place when the MMSC vendors were swapped out over 4 years ago and it's following RFC messaging standards. The old vendor used to route it down a static gateway but if that gateway was down or failed, nobody got their messages! For modern gateway redundancy, the messages need to round-robin to the nearest available SMS gateway and so the messages are not hashing to a specific exit.