Cannot send email: 550 message submission is for authorised users only

Symptoms

When sending an email through a smarthost, the message will not be sent, and you will encounter a message similar to the following.

550 message submission is for authorised users only

Description

The 550 message is sent from the smarthost's server and occurs when there is an issue at the smarthost server or a problem with the credentials used to access the smarthost.

Workaround

You will have to contact the smarthost provider to find out the cause and if any action needs to be taken on your server.