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Microsoft has complete documentation on this.<br />
<a href="https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/use-dmarc-to-validate-email" rel="ugc">https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/use-dmarc-to-validate-email</a><br />
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<i>We Use Exchange online for most of our mail, but we also have an on-premise SMTP server for routing mails from our web servers</i> they should go through office 365.. just identify the ipv4 address in your spf record<br />
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<i>And we are using a cloud service for our newsletter service</i> request a dkim record from the cloud service.
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Holger Nielsen

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You can enable DKIM on onpremise.<br />
It will not effect your O365 mail flow. <br />
But you may have to install DKIM signing software in Exchange server and add a selector and publish in external DNS
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We are planning to set up DKIM for a mail domain, example.dk.<br />
I can find a lot of good advice on the Internet. But there's one question I can't find an answer to.<br />
We Use Exchange online for most of our mail, but we also have an on-premise SMTP server for routing mails from our webservers. And we are using a cloud service for our news letter service. How can DKIM be set up when there's multiple systems sending mail from our example.dk domain?<br />
All the guides I have found on the Internet are about a setup with only one mail system.<br />
<br />
<br />
Best regards Holger Nielsen
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We are planning to set up DKIM for a mail domain, example.dk.
I can find a lot of good advice on the Internet. But there's one question I can't find an answer to.
We Use Exchange online for most of our mail, but we also have an on-premise SMTP server for routing mails from our webservers. And we are using a cloud service for our news letter service. How can DKIM be set up when there's multiple systems sending mail from our example.dk domain?
All the guides I have found on the Internet are about a setup with only one mail system.


Best regards Holger Nielsen

DKIM with multiple mail systems

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.