cPanel® & DirectAdmin plugins have this automatically enabled.
Domain spoofing poses a serious threat to email integrity and security. This issue arises when attackers impersonate a domain they don’t own, often for malicious purposes like phishing or distributing malware. To counteract domain spoofing, three security measures—DKIM, SPF, and DMARC—operate together on the open internet to authenticate emails and prevent fraudulent activities. These protective layers are effective on the open internet. However, in a closed, multi-tenant email delivery service like JetEmail, where multiple domains use a shared IP space for sending emails, relying solely on SPF to secure your domain against spoofing is inadequate. Domain Lockdown enables you to specify which JetEmail SMTP users are authorized to send emails from your domain by setting your preferences in a DNS TXT record.

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DNS Record

Name: _jm.domain.com TTL: 14400 Value: 
v=jm1 auth=jetemail

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Finding your SMTP username.

You can find your SMTP username by looking through the internet / email headers for X-JetEmail-User. It should look like this: 
X-JetEmail-User: jetemail
Replacing jetemail with your SMTP username

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Authenticating multiple accounts

You can add multiple auth headers for multiple accounts, for example: 
v=jm1 auth=jetemail auth=jetemail1 auth=jetemail2

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Blocked Messages

If your message does not authenticate, the mail will bounce with a message similar to this:

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Further Assistance

You can contact our support team any time should you have any questions or issues related to Domain Lockdown.