I was subscribing to a mailing list that uses the Sympa software. This works like most of these systems - you give it your email address and it sends a confirmation email to that address with a link in it to click on to prove that you have control of that email address. This prevents someone from subscribing for another person.

I got the confirmation email right away and when I clicked on the link, I went to the web site as expected, but it gave me the message "Sorry, this operation can't be performed... The validation link has already been validated from host x.x.x.x. If you did not perform this validation, please report this confidentiality issue to your mail services administrator."

I thought "who has been reading my e-mail and clicking on links for me?" Well, that IP address belongs to Barracuda Networks, Inc. The email filtering software activated the link while checking it for malware.