To be honest, this is a really tough one. I run into this all the time. On the various email lists that I manage, I’ll end up getting a bounce message for an email that’s not on my list.
And while I have some ideas as to why, there’s little I can do.
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Automatically forwarded email
I suspect that someone who is on your list and getting your emails is automatically forwarding your email invitation to this other person.
In my case, I have little information to help me determine how or why this is happening. Often, I can’t identify which email address is the one that is doing the automatic forwarding. This is very painful, especially if someone is upset on the receiving end.
Now, the fact that you’re in contact with the person who is receiving the email gives you a little bit of hope. To figure out why this is happening, you need to ask that individual to send you a full copy of the email that he received including the detailed headers of the email.
How detailed email headers help
What’s neat about an automatically forwarded email like this one is that the headers should contain detailed information that’s usually hidden by the email system. Using this, you can actually track step by step the path that the email took as it left your server and landed in his inbox. That way, you can find out either who sent it to him or what service or server actually forwarded that email.
The only downside is that you need the upset person at the other end to cooperate with you and send you the email with the detailed headers. Hopefully, he’ll understand that by helping you, he’s actually helping himself. Without that information, I’m afraid that there’s little that can be done.