puzzling issue: recipient prompted for password when replying

[1415bhwong]

ONE OF OUR VENDORS FEEDBACK THAT WHENEVER HE ATTEMPTS TO REPLY OUR EMAIL, HIS OUTLOOK WILL PROMPT HIM TO ENTER A PASSWORD FOR OUR ZIMBRA SERVER. ANY IDEA WHY THIS IS HAPPENING WHEN HE IS NOT EVEN USING OUR ZIMBRA SERVER TO SEND HIS EMAIL??? (SEE ATTACHED)

image003.jpg

[kruon]

Maybe the sender's email signature contains some image stored on the zimbra users briefcase or something?
"View source" might reveal what is linking to the zimbra server from that mail.