If You Encounter Delays or Undelivered SMS with T-Mobile Using External Delivery

T-Mobile operates with a SPAM filter which often affects services that use Email to SMS gateways (the free External Delivery uses Email to SMS gateways of your carrier). As a result, T-Mobile users often see delays or undelivered SMS messages if the sending AWS server’s IP address is on the SPAM filter T-Mobile uses.

What you might see if this occurs

If you find that a Rule was triggered in the iMonnit Online Portal but did not receive an SMS, you will want to investigate the Rule escalation. This will help you determine if the SMS was sent:

As a result, delivery of the free External Delivery SMS is often delayed or not delivered. This is not a function of the Monnit system but rather with the filtering the carrier applies. Therefore this functionality cannot be adjusted on the Monnit side. If you persistently encounter this, an alternative would be to purchase Notification Credits and use the paid Direct Delivery feature, which will deliver reliable direct SMS messages not using the Email to SMS gateway of the carrier.

If you find that the SMS alert was indeed triggered and sent in the iMonnit Online Portal, you will want to look at the Status column of the Rule History. If you see “SMS Sent via SMTP” (most commonly), “Soft Bounce”, or “Hard Bounce”, you may be experiencing this issue. In some circumstances, the messages may be delivered some matter of hours or days later. It is also worth noting that this issue will be encountered 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. As AWS circulates the IP addresses from which these email servers send, it is possible for Monnit to be sending (for a time) from an IP address that had previously been reported to T-Mobile as a SPAM address. As a result, if Monnit happens to be using one of those servers for that day, it is possible for the message to be blocked while iMonnit is sending from that specific server address. However, once the IP addresses are recycled, iMonnit may start sending from a different IP (sometimes the next day) which is not on T-Mobile’s SPAM list. In that case, it is possible for the SMS to be delivered promptly. That is to say that this issue can be very intermittent.

Using Direct SMS

You can certainly pursue the use of the more reliable paid Direct SMS if you encounter this persistently. You can change your User’s Notification Details in the iMonnit Online Portal.

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