Carrier rate limiting on SMS campaigns

Carrier rate limiting happens when mobile carriers slow down or block part of an SMS send because too many messages were sent too quickly from the same number. This warning appears when SMS delivery provider reports rate-limit errors back to Cymbal during your send.

Why this happens

Mobile carriers set strict per-second throughput limits for each phone number. These limits vary by carrier and number type (long code, toll-free, short code). When a campaign exceeds those limits, carriers may:

• Delay delivery (throttling)

• Queue messages temporarily

• Reject messages with rate-limit or filtering errors

SMS delivery provider will retry throttled messages when the carrier allows it, but messages that carriers explicitly reject for rate limiting or filtering are usually not retried.

What the warning means

Cymbal shows Carrier Rate Limiting when:

• SMS delivery provider returns carrier errors indicating that the send rate was too fast.

• The number displayed is how many message segments were rejected.

• Delivered or successfully queued messages are not included in the count.

Rejected messages are generally treated as final failures because the carrier blocked them at send time.

How to reduce carrier rate limiting

Send in smaller batches. Large, fast bursts of traffic are the most common cause of rate limiting. Breaking a send into smaller segments reduces the likelihood of hitting carrier thresholds.

Keep messages short. Longer messages are split into multiple SMS segments, and each segment counts separately toward carrier throughput. Shorter messages reduce the total number of segments sent per second.

Avoid rapid, repeated sends. Sending multiple campaigns or automation texts to the same audience in a short window increases the chance of being throttled or filtered.

Follow compliance guidelines. Carriers filter traffic more aggressively when messages look spam-like. Include clear sender identification and opt-out language such as “STOP to stop.”

Do blocked messages get retried?

Not typically. If a carrier rejects a message for rate limiting or filtering, SMS delivery provider usually cannot retry it. If the message was time-sensitive, you may need to resend a smaller batch or follow up via email.

If you’re seeing repeated rate limiting on normal sends, contact Cymbal Support with the campaign URL so we can review delivery logs and help you adjust sending patterns.