DMARC is a system that helps email operators to check the authenticity of email messages and to prevent email spoofing. DMARC has some rules that the email messages have to follow to pass the check. If the email messages do not follow the rules, they may be rejected or marked as suspicious.

DMARC uses two methods, SPF and DKIM, to verify the domain names of email senders. These methods have their own limitations, which also affect DMARC.

Some email messages may go through some changes or modifications by other services, such as mailing lists. These changes may make the SPF or DKIM verification fail, or make the domain names mismatch. These changes may follow the email standards, but they will still make the DMARC check fail.

Also, some email messages may be sent by other parties that are allowed by the domain owner, but they may not use the same domain names as the domain owner. This will also make the DMARC check fail.

There are some more issues that are related to how DMARC works with DKIM. You can read more about them in [DKIM-LISTS], especially Section 5.2.

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