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We are looking into setting up DKIM to improve email deliverability and conform better to best practices for email. We currently have two servers that can send email for a domain - one is CentOS running postfix and the other is SBS 2008 running Exchange 2007.

It is more important for us to set up DKIM on the server with postfix, which has higher, script-generated mail volumes and has had deliverability issues; this looks to be relatively straightforward. It looks like setting it up on Exchange 2007 is much more complex, and this side is not as crucial, as we have lower, more personal mail volumes and have not had deliverability issues.

I'm trying to understand how DKIM works but I'm not experienced yet. My question is this: if we set up DKIM only on the postfix server and add the DNS records, could this negatively affect mail from the Exchange server? Will email providers be expecting DKIM and not see it and be more likely to reject messages compared to when they are not expecting DKIM at all?

Basically I'm trying to weigh the risk of messing up email by screwing up the Exchange box trying to set up tricky DKIM vs. the risk of messing up email by telling email providers to expect DKIM and then not seeing it on one of our servers.

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Short version:

AFAIK, the big provider track email reputation based on client IP address, not domain. In fact: there are many DNSBL for track IP address but not for domain.


When an email from your exchange server arrive on destination, it will checks whether the email have the DKIM signature. Because it doesn't, the destination will lower the reputation score of the email and client IP address. So, your chance being rejected is same with other email without DKIM signature.

Now, when an email from postfix server arrived, it has valid DKIM signature. It will increase the reputation score of postfix IP address.


Adding DKIM signature only for postfix (probably) doesn't make your Exchange server loss reputation. Instead, it will increase the reputation of Postfix email client. For destination view, Exchange server will have steady reputation score, while the postfix server will have higher reputation score

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