When a message is said to have passed DKIM validation checks, that means that the validator has been able to ascertain that the message was not altered between the time that the DKIM-Signature header was inserted and the message made it to its destination. Unlike SPF, DKIM is a content-based authentication protocol. Since it's quite likely that the domain owner will only have authorized server A to use its domain, intermediary B's IP address won't be found in the domain's SPF record, and so SPF will fail. When that message arrives at destination C, the connecting IP address will be that of intermediary B. This works just fine for direct mail, but problems can occur when server A originates a message that passes through intermediary B on its way to destination C. When the mail arrives at its destination, the SPF record of the domain in the Return-Path is checked, and if the contents of that record authorize the connecting IP to use the domain, then SPF passes for that message. With SPF, a domain owner declares the server and networks that are authorized to send mail using that domain. SPF is what's known as a path-based authentication protocol. Unfortunately, forwarders can cause one or both of these checks to fail for mail that would easily pass if it were routed directly from its source to its destination. A DMARC pass verdict requires that only one of the two pass, but that the passing protocol(s) also possess a quality known as "domain alignment", where the checked domain is similar, or in some cases identical, to the domain in the visible From header. How Can Forwarders Present Challenges for DMARC?ĭMARC relies on two underlying authentication protocols - SPF and DKIM. The point with all of these is that the forwarding must be automated in order to potentially cause issues with DMARC. Regular mailbox holders who control two or more mailboxes and want all mail from one automatically forwarded to another.College alumni addresses, such as which forwards automatically to Anonymous mailing services, used by people who don't want to provide their real email address in sign up forms.While there are many different permutations of this sub-class of forwarder, three of the most frequently seen are:
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