AT&T has a strong business interest to protect our customers from spam. We have systems in our network to reduce spam. If your program send messages through our network that look like spam, our systems may block them.
Eavesdropping protection for SMS messages on AT&T's wireless network use the same A5 encryption algorithm used for GSM voice communications.
If you are an Enterprise Alliance or Corporate Customer using SMS with your application, consider using Enterprise Paging rather than SMTP. If this is not feasible, work with your AT&T business development or account manager so the AT&T wireless network does not treat your business-critical messages as spam.