Build Number:129787

When a user set up a vacation rule. It replies to internal users fine.
But there's an issue with replying to external users. The reply shows as sent in the users sent items.
But the external sender does not get the reply.
My Firewall SMTP logs shows this error for the reply:
warning: Illegal address syntax from unknown[] in MAIL command: <Mailer-Daemon@>

The user can send to any external emails with no issues.
This happens with Multiple users.

Any idea what the problem is?