Windows Security (Zimbra)

This only happens for one person, and every time I reply back to his email I get a pop up with a Login screen (see attachment). This person has a email address and I can email other people in his company with no issues.

Has anyone else encountered this issue?

It might be related to some linked content in his email, but your best bet is to open a support ticket and get some qualified reply and hopefully an immediate solution.

Thanks I will keep in mind about opening a support Ticket.
I also email back the person with the email to let him know that this keeps popping up. I will give it some time and see if his IT people can fix the issue, as you say it could be something linked on his email, and see if this login keeps popping up when I reply to his email’s