The "550 command rejected due to Sender ID validation failure" SMTP non-delivery report (NDR) indicates that the email you attempted to send was rejected by the recipient's email system due to a Sender ID validation failure.

Sender ID is a security protocol used by some email systems to verify the authenticity of the sender's email domain. It works by checking the DNS (Domain Name System) records of the sender's domain to ensure that the sending IP address is authorized to send email on behalf of that domain.

If the Sender ID check fails, it means that the recipient's email system doesn't recognize the sender's domain as a trusted source for email. This can happen for a variety of reasons, such as:

- The sender's domain doesn't have a valid SPF (Sender Policy Framework) record in its DNS settings.
- The SPF record doesn't include the IP address or range used to send the email.
- The IP address or range used to send the email has been flagged as suspicious or known to send spam.

To resolve this issue, you should contact the IT department of the recipient's email system to determine why your email was rejected and how to fix the Sender ID validation failure. In the meantime, you can also try sending the email from a different email account or using a different email service.
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