Determinig if SMTP Relay is sending duplicate messages

Ian Crowl 1 Reputation point
2021-11-01T14:19:19.217+00:00

We have a setup where one of our inhouse apps will create emails to customers about training courses that need to be complete. We use scheduled tasks on our "utility" server to trigger this app to generate the data, it drops .eml files to an Azure File Storage Storage account. We have a box dedicated as our SMTP relay server. this box has a scheduled task that will run a script to pull messages from the SA to the drop folder on the local box. The SMTP relay will then process the messages in the drop folder and send them to mailgun to be sent to the customers.

We have been getting reports that duplicate messages have been received by customers. However, this it intermitted. We have our internal customer support team relaying reports of the dupe emails when they occur we have enabled logging on the SMTP relay server and can see if a message is processed. Since enabling the logging, we have received one case and we were able to see in the logs that the recipient does show in the logs twice. It is important to note that the way our inhouse app works is, it will only notify the customer once per day, If the customers have more than one training course due, it will include all of the required trainings in the same message. To reiterate, the customer will only receive a single message each day.

We believe we have concluded that mailgun is not duplicating the messages, but now need to determine if SMTP is duplicating the messages or if the inhouse app is duplicating the messages.
I am asking for help to create a solution to finding where the duplicate messages are being created. I understand you can not help in regards to how the inhouse app works but maybe there is something we can configure from the SMTP relay server in IIS 6.0. Any debugging, additional logging, or monitoring of the drop folder, etc... would be very helpful to determine if the issue is SMTP relay or not.

Open to all suggestions.
Thanks
Ian

Internet Information Services
Windows Server
Windows Server
A family of Microsoft server operating systems that support enterprise-level management, data storage, applications, and communications.
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