553 sorry, that domain isn't allowed to be relayed thru this MTA

When sending mail, I receive the following error:

'553 sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1)'

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This error usually means that authentication was not supplied or had
expired. This will usually happen when you send mail and haven't
checked mail in the last 30 minutes.

To avoid this error, you would want to ensure that you check mail
before sending. Doing so puts you in the authenticated list and can
then send. However, a better option is to use SMTP authentication.
Please check our knowledgebase article on using SMTP authentication. In
outlook you would set this in more settings.