Skip to main content

Carrier Blocking and Spam Detection

The phone carriers (ATT, Sprint, Verizon, etc) all have their own distinct rules and algorithms for determining if outbound messages are “spammy” in nature. Those algorithms take into account message content, outbound rate of delivery, number of customers that opt-out following the receipt of a message, and the rate of bi-directional engagement on an SMS number.

If carriers detect “spammy content” they may deny delivery of messages. In extraordinary circumstances, they may even blacklist a phone number and deny delivery of all outbound messages on a specified SMS channel to their customer base.

The carriers are all independent entities. Being rate-limited or marked as spammy on the Verizon network doesn’t necessarily mean that Sprint will mark the same messages as spammy.

Please note that some carriers will incorrectly and purposefully report to that messages that are carrier-blocked were actually delivered. may have very little visibility into the viability of outbound messages that are carrier blocked.

Messages delivered via unregistered long-code (10 digit phone numbers) are much more susceptible to spam filtering than messages delivered via short code (5 or 6 digit numbers) and toll free numbers. Message delivery via fully registered 10DLC will be extremely strong, rivaling that of short code and toll free.

Keys to Minimizing Carrier Blocking

  • If possible, target smaller lists of customers with the same message. Instead of sending the same message to 100,000 folks, send slightly modified messages to 10 groups of 10000 by using the A/B Variant functionality in Broadcasts. Your sales engineer may be able to help configure rotation through a set of outbound messages.

  • Keep messages short -- shorter than 160 characters if at all possible. (Sprint, in particular, seems to flag messages longer than 160 characters as spam.)

  • Do not use aggressive language, curse words, or threatening language.

  • Don’t use a lot of capital letters except to start sentences. Messages sent such as “CLICK HERE NOW” are very likely to get spam blocked.

  • Encourage customers to interact with the message. The more people that reply and engage in bi-directional (two-way) conversations, the lower your “spam score” will be.

  • If you are sending a link in the message, make sure that the domain is a well known primary domain. Do not use redirecting links (for example as those are often a key indicator for the spam detection algorithms. Links should appear at the end of the message and not in the message body if possible. Embedded links get flagged more often than links that appear at the end of the message. The Prompt Smart Link service should be used for sending trackable, custom links. If you will be sending a lot of links in your messages, consider getting a custom Prompt Smart Link -- we can even help you send links from your own primary domain. Contact customer success for more details.

  • Providing opt out language in your message body may lead to higher delivery (but may also lead to more opt outs).

Rate Limiting

In order to maximize successful delivery of SMS, will limit outbound text messages sent via bulk messaging to approximately 80% of the maximum speed granted by the carriers for your organization's specific use case. This speed is determined by The Campaign Registry for 10DLC. For MMS, the platform will send 1 message per channel every second.

If you have a very high number of customers or a compressed timeframe for delivery, we suggest segmenting the audience into multiple lists and performing outbound texting over multiple SMS outbound numbers while varying the message body for each segment.

Please note that delivery via toll free caps our at 1 message per second. Short code will send at approximately 20 segments per second.

Additional Resources

Please contact client services if you have any questions about best practices, rate limiting, land-line texting, or carrier blocking.