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Javier Tarí

Duplicated SMTP messages

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I'm having a strange issue sending messages through TSMTP

The emails are sent to a fixed destination, and CC'd to the sender email

Although I only send them once from program, About 5% of them are sent twice

To increase my confussion, I've added as BCC my own email, and that email doesn't get duplicates


So I'm quite lost




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Indy's TIdSMTP does not send duplicate emails, so assuming that:

  • you are not calling TIdSMTP.Send() multiple times
  • do not have the same recipient listed multiple times in the TIdMessage's Recipients/CCList/BCCList properties
  • one of your recipients does not forward the email to another of your recipients

Then this duplicating behavior would have to be happening on the server's end, not on Indy's end.


If you (or a recipient) looks at the raw source data of the duplicate emails in your/their inbox, are their headers EXACTLY the same?  Right down to their reported delivery route, timestamps, etc?

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Thanks for answering, Remy; I'm sure it's not Indy's fault, but I'm quite lost


All and every email is sent to exactly the same recipients: the email addresses are used by an automated system, always from the same "From" to the same "To", CC" and "BCC"

All the emails are sent successfully, and arrive successfully. Some of them (5%-10%) are received twice, and never more than twice

  • I call TIdSMTP.Send once
  • The source of the repeated emails, at least as seen on Thunderbird "source code" message option, is exactly the same, even compared with a text comparer

On the source I can see some IDs (as I said, exactly the same on both emails):

X-Cyrus-Session-Id: llcc507-a-18474-1651934775-1-2233586116162072206

ESMTP id 4KwVcR2Dvdz30vg

ESMTPS id 4KwVcQ5F2Lz3072

ESMTPA id 4KwVcQ1gtSz2t2D


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Then you will have to talk to your email provider, see if they can trace the source of the duplication while you send test emails to them.

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