And they are definitely responsible for the systems they are notified for.
We use the SMTP gateways to not overwhelm the services. (10digitnum@vtext.com... etc... 1234567890@rogers.com) and we deliver singly, not in batch mode.
Verizon delivers in mere seconds. AT&T delays up to 30 minutes. T-Mobile doesn't delay more than a minute. Rogers is also near instantaneous.
Dunno. But this is not that tough.