DomainKeys Identified Mail (see https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail) a technique for authenticating email, to ensure it has not been 'spoofed' (i.e. sent from a different domain than that used in the 'sender' field). DKIM uses cryptographic methods, and is stored in TXT record. Even if you outsource the handling of your mail, you will need to configure a DKIM and SPF record on your domain. See also SPF.
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