If You Encounter Delays or Undelivered SMS with External Delivery

Some wireless carriers (including AT&T, T-Mobile, Verizon, and others) operate with a filter that uses the SORBS list. The filter affects services that use email-to-SMS gateways (the free External Delivery uses your carrier’s email-to-SMS gateways). As a result, the free SMS messages sent to these carrier users can experience delays or not be delivered.

What You Might See If This Occurs

If you find a Rule triggered in iMonnit but don’t receive an SMS, investigate the Rule escalation. This will help you determine if the SMS was sent:

This isn’t a function of iMonnit but instead of the carriers’ SPAM filtering operations. Therefore, this functionality cannot be adjusted on the Monnit side. You can avoid this issue by purchasing Notification Credits and using the paid Direct Delivery feature, which will deliver reliable direct SMS messages without using the carrier’s email-to-SMS gateway.

If the SMS alert was triggered and sent from iMonnit, you can look at the Status column of the Rule History. If you see “SMS Sent via SMTP” (most common), “Soft Bounce,” or “Hard Bounce,” you may be experiencing this issue. In some circumstances, the messages may be delivered hours or days later. You may also experience this issue intermittently.

Rules - Sent SMS
Rules - Sent SMS


Monnit uses AWS servers to originate the SMTP email messages sent to the carrier’s email-to-SMS gateway. AWS uses a pool of email servers to forward these notifications. When Monnit sends the email, AWS selects which email server from the pool will deliver the email to the carrier. Your notifications can be sent from a server address on a SORBS list. As a result, if AWS happens to select that server, that message may be blocked.

Our best estimation is that AWS implements sticky load balancing that causes specific recipient addresses to be directed through the same AWS server for a period of time. This estimation is based on observing one recipient address consistently blocked for some time while other users are unaffected. As a result, the delivery of the free External Delivery can be intermittent.

Using Direct SMS

If you encounter this persistently, you can configure the more reliable paid Direct SMS. To apply this configuration, you can change your User’s Notification Details in the iMonnit and purchase Notification Credits.

Direct SMS doesn’t use email-to-SMS bridges. This paid service injects the message directly into the SMS network, providing higher reliability.

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