Letterbook
All docs

Letterbook Docs

What is DKIM? Why do we need it?

Learn how DKIM helps receiving mail servers trust messages sent by Letterbook on your behalf.

Last updated June 22, 2026

What DKIM means

DKIM stands for DomainKeys Identified Mail. It is an email authentication standard that lets a sending service attach a cryptographic signature to an outbound message.

When Letterbook sends a reply from your support address, the receiving mail server can check that signature against a public DNS record on your domain. If the signature matches, the receiver has evidence that the message was authorized by the domain owner and was not modified after it was sent.

Why Letterbook needs DKIM

Letterbook sends replies on behalf of your support address. Without DKIM, receiving mail servers have less evidence that those replies are legitimate. That can cause messages to look suspicious, land in spam, or fail authentication checks on stricter domains.

DKIM helps with three things:

  • Trust: receiving servers can verify that Letterbook is allowed to sign mail for your domain.
  • Deliverability: authenticated messages are less likely to be treated as spoofed mail.
  • Brand consistency: customers see replies from your support address instead of a generic sender.

How DKIM works with DNS

DKIM uses a pair of keys:

  • A private key used by the sending service to sign outbound messages.
  • A public key published in your domain's DNS records.

Letterbook handles the private signing side. You publish the DNS records Letterbook provides so receiving mail servers can verify the signature.

DKIM, SPF, and DMARC

DKIM is one part of email authentication.

  • DKIM verifies that a message was signed by an approved sender for the domain.
  • SPF lists which mail servers are allowed to send for the domain.
  • DMARC tells receivers what to do when authentication checks fail.

For reliable sending, all three should be aligned with the domain customers see in the sender address.

Auto configure versus manual setup

If your DNS is hosted in Cloudflare, Letterbook can use Domain Connect to request the required records automatically. You approve the DNS changes in Cloudflare, then Letterbook verifies the domain.

If your DNS is hosted somewhere else, copy the records from Letterbook and add them manually in your DNS provider.

Related guide

Return to Enable sending from your support address to configure DKIM for Letterbook.