From an end user account in our domain, our Exchange Server repeatedly attempts to send an email to a recipient with the following (modified) address: email@example.com. (The web.de domain name is correct.) The end user continually receives a bounceback back message, as follows:
web.de #550 Requested action not taken: mailbox unavailable ##
Earlier in the day an email FROM a sender in the "web.de" domain was detected by our firewall as containing spam. It stripped the spam but allowed it to continue to our Exchange Server, which allowed it to be delivered to the end user's mailbox. At that point, the situation began as described in the first paragraph.
Thus far, the end user has counted 55 instances of the "Undeliverable" message from our Exchange Server. The strange part of this is that the "firstname.lastname@example.org" email address changes with every bounceback message. The only constant is the domain remains the same, i.e., web.de.
We've run Anti-Spam and Anti-Virus scans on the end user's pc. It is clean.