Im having a weird issue.

A customer of mine is trying to respond to an email. When he attempts this he gets a pop up window that asks for the credentials (un and pw) of the domain he is trying to email. The first message asks for the username and password of the mail server he is emailing. If you enter bad creds or hit cancel a new window pops up. The second one says "A username and password is being requested by email.domain.com. They site says: "email.domain.com" Where email.domain.com is the MX of their mail server.

So it looks like its their email server asking for the creds, but why? I sent a test email from an exchange server and it went through with no issues.

My customer says this is the only time this issue has happened.

Any Suggestions? Im new to Zimbra and at a loss on what might be causing this.