Mail Flow

Email Security Foundations #05 – DKIM deep dive: signing, selectors and how verification works

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This post is part of the Email Security Foundations series on MSDigest.net. What DKIM actually does DKIM stands for DomainKeys Identified Mail. Where SPF checks where a message came from, DKIM proves that the message hasn’t been changed while it was in transit. When you send an email, your mail server adds a cryptographic signature to the message. When the receiving server gets it, it looks up your public key in DNS and uses that …

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Is Your On-Premise Exchange Server Ready for March 22nd 2026?

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If you run an on-premises Exchange Server or any Windows-based SMTP relay that sends mail to or receives mail from Exchange Online, then you might have a hard deadline coming up: March 22, 2026. Miss it, and you can risk breaking your mail flow, if the Root Certificates used by Exchange is not updated. What Is Changing Microsoft Exchange Online is switching its TLS certificates to the DigiCert Global Root G2 certificate authority. This is …

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How to fix the error: 550 5.7.520 Access denied, Your organization does not allow external forwarding

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I have often seen this error in scenarios with either multiple tenants or if an organization, has a need to do automatic forwarding of e-mails to specific recipients outside the organization (tenant). The error can be found during troubleshooting the message logs, on why a specific e-mail did not arrive at its destination, or if the end users got the error when their e-mail bounces. Remote Server returned ‘550 5.7.520 Access denied, Your organization does …

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