DKIM (DomainKeys Identified Mail) allows senders to associate a domain name with an email message, thus vouching for its authenticity. This is done by "signing" the email with a digital signature, a field that is added to the message's header. The recipient MTA can verify the DKIM signature, which gives users some security knowing that the email did actually originate from the listed domain, and that it has not been modified since it was sent.
Use this tool to lookup and validate your DMARC record.
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