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Asked: 2026-06-07T07:59:18+00:00

Problem steps : My company sends mail to external partners (using .Net SmtpClient), Some

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Problem steps :

  • My company sends mail to external partners (using .Net SmtpClient),
  • Some of them have set up “transparent forwarding” on our mails to some of their co-workers,
  • Those co-workers receive our mails as if we have send them directly to them (no “on behalf of”, no classical “forward” from the original “To” mailbox), without any clue of which mail box has transparently forwarded them the mail.
    (In fact, smtp conversation traces in the mail properties allows to find it, but most users are not tech savvy enough to be able to access such information, and we have not the knowledge for explaining them how to do for all the varieties of mail clients our partners may use),
  • Some of those co-workers complains to us that we are spamming them. And if their are unable to transmit us the offending mail untouched (usually as an attachment, preserving the smtp conversation traces), we are helpless in determining by which way they have got our mailing which was not originally directed to them.

Is there a way using SmtpClient to set-up some properties which forbids “transparent forwarding” ?

I do not want to forbid normal forwarding or copying as in How do I prevent Lotus Notes users from forwarding or copying a message sent via System.Net.Mail? question.
I just do want to forbid that “transparent forwarding” (which is IMHO a very bad feature of some mail systems like MS Exchange, which in its rules options gives two way of forwarding, one being “transparent”).

As this feature looks to me as being not compliant to smtp RFC, I really fear the answer is no or almost no.
But if your are aware of some custom header for forbidding this on some of those mail systems who do offer that transparent forwarding feature, it may still be of some help to me if their are settable using SmtpClient.

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1 Answer

  1. Editorial Team

    If you are trying to solve the problem of not knowing who the original recipient of the message is, then try adding a line in the body of the message something like this…

    “This message was intended for (insert recipient here).”

    You can make it visible so the user can see who the original recipient was, or (if you can) put it in super small font so you can use it for troubleshooting/spam claim illustration purposes.

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