Knowledge Base Article

Email Error: 553

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

Solution:
Due to increased security on many public email servers, the server requires that

Go to Microsoft Outlook, select the Tools menu and then Email Accounts.  Select the relevant email account, and ensure that the setting "My outgoing server (SMTP) requires authentication" is selected (see below) from the More Settings option and on the Outgoing Server tab (as indicated below).

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