I would second this. It's extremely difficult to verify without obtaining an original of the email which initiated it, however.
Sometimes it helps to merge wording like this email was sent to 'email
address' in the emails
That way you can track such kind of email forwarding
If it is not desired merge email address in a html comments code part of
the footer.
We did that in past
Rajesh
On Dec 7, 2015 9:35 AM, "Kenny Elkington" <marketingnation@marketo.com>
If you're not able to find any similar addresses, then, as Kenny says below, the only other option would be to get the lead to save a copy of the email they're receiving, attach it to a message, and send it to you. They can't just forward it, as you need to have the headers intact from when it was sent to them from Marketo. Then we can look at those headers to see where it's coming from.
John