Yes, that's true. In my experience doing batch sms-emailing (with relatively small batches -- several hundred at a time only), we would take use two strategies

Yes, that's true. In my experience doing batch sms-emailing (with relatively small batches -- several hundred at a time only), we would take use two strategies: 1 -- queue the messages and send them slowly, maybe between 5-25 every minute. and 2 -- distribute across carriers. Group the users by carrier and then for each queued batch take from as many of the different carriers as possible, thus limiting the number of times you sent an email over any specific carrier's gateway in a given queued clump of messages. This seemed to work but it was hard to guarantee message delivery.