PHP contact form giving 550 5.1.1, “User unknown”

Posted July 14, 2014 2.4k views

Hi, I run an Ubuntu server with Apache2. I have a PHP contact form in The contact form is configured to send messages using SendMail to ( is a Zoho hosted email).

A few hours ago I was being able to send messages via contact form, but it suddenly stopped to work and started returning this error:… User unknown

Any idea?

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Does sending an email to from your personal email address (Gmail, Hotmail, etc.) work?