Email Response Message - Mailbox Unavailble

When sending an email whether through a mailbox, mail application (IE: Outlook, Thunderbird), or mail form several returned messages are received with the following message:
System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable.
The most common cause for this issue is because SMTP Authentication is not enabled for the mail domain sending the email.
Please contact your SMTP server provider to request that SMTP Authentication be enabled for the domain.

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