Over the last few hours, a co-worker has received 5 emails, from different IP addresses, with the subject line: "Undeliverable: any text here".
Related IP: 22.214.171.124
Related IP: 126.96.36.199
Related IP: 188.8.131.52
Related IP: 184.108.40.206
Related IP: 220.127.116.11
When I review the Exchange Server logs, there are no entries of her sending any email to the recipients.
When I look at the complete Header of the emails, I see they originated in a country other than the US, then are routed through mail Servers in the US, then to her Inbox.
Her email address is the "Reply To" address.
How can I identify the problem here?