DKIM standard provides a mechanism to encrypt your email message in transit and protects the senders and recipients from spam, spoofing, and phishing. This standard uses an encryption key and a digital signature. ZeptoMail makes it mandatory to configure DKIM to protect your email content from alteration by spammers. The email content will be encrypted before it is transmitted.
Article on DKIM - Link
Steps to add DKIM records - Link
Queries - support@zeptomail.com