Email Error #5.1.2 sending e-mail to external recipient

SMTP 5.1.2 Error when sending an email, What does it mean?

Solution:SMTP 5.1.2 Error message indicates the domain name is incorrect or doesn’t exist in the address the message was sent to.
Verify again whether the e-mail address is correct and try to change the Outlook mode to the online mode if it does not.