The "550 command rejected due to Sender ID validation failure" SMTP Non-Delivery Report (NDR) indicates that the receiving mail server (in this case, Outlook.com) has rejected the email because it failed the Sender ID validation check. Sender ID is a technology that checks if the domain name of the sender's email address matches the domain name of the sender's mail server. This helps to prevent email spoofing, which is when spammers and phishers falsify the header information of an email to make it look like it came from a legitimate source.

If you received this NDR, it means that the email you attempted to send was rejected because the domain name in your email's "From" field does not match the domain name of your mail server. To resolve this issue, you may need to update your Sender ID policy or contact your email service provider to make sure that your email is being sent from a domain that is authorized to send emails on behalf of your organization.
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