PHP contact form getting "User unknown error" when trying to send message to my hosted Zoho account

PHP contact form getting "User unknown error" when trying to send message to my hosted Zoho account

Hi, I run an Ubuntu server with Apache2. I have a PHP contact form in domain1.com. The contact form is configured to send messages using SendMail to my Zoho hosted email account: info@domain2.com.

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:

<info@domain2.com>... User unknown


Any idea?