Carriers (AT&T, Verizon, T-Mobile, etc) have a vested interest in keeping spam off their network and protecting their customers. They employ a variety of tactics to detect messages which violate their policies.
Sending spam via our system or content purposely obfuscated to get around spam filters is expressly prohibited, and will result in banning your account and possible prosecution.
Avoid looking like you're selling something
Don't include sex, hate, alcohol, firearms, or tobacco in your messages.
Avoid "spammy" content - e.g., "FREE! BUY NOW! ONLY $10!"
Avoid URL shorteners
Including links generally will look like spam.
Keep the message short
If possible, keep the content to less than 160 characters.
Minimize repetitive messages
Don't re-send the same message to lists regularly. Your messages should be varied and unique.