Definition
DKIM stands for DomainKeys Identified Mail and is defined in RFC 6376. When your mail server or sending platform sends a message, it signs selected headers and the body with a private key. The signature travels in a DKIM-Signature header.
The matching public key lives in DNS as a TXT record at selector._domainkey.yourdomain.com. The selector is a label chosen by whoever set up signing, so one domain can have several keys, for example one for Google Workspace and one for a marketing platform. The receiving server reads the selector and domain from the signature, fetches the public key and checks that the message still matches.
If someone edits the signed parts of the message in transit, the check fails.
Why it matters
DKIM is one of the three records receivers use to decide whether mail claiming to be from your domain is genuine. Gmail and Yahoo have required authentication for bulk senders since February 2024: SPF and DKIM, plus a DMARC record, with the visible From domain aligned to SPF or DKIM. Microsoft announced similar rules for high-volume senders to Outlook.com in 2025.
DKIM also carries reputation. Because the signature is tied to your domain rather than an IP address, mailbox providers can build a history for your domain that follows you across sending services. That matters for cold email teams who rotate mailboxes and tools.
A common failure is signing with the platform's shared domain instead of your own. The signature passes, but it does not align with your From address, so it does nothing for DMARC.
How SimpleCold handles it
DKIM is about the sender, so it is not part of verifying a recipient's address. SimpleCold's email verification checks the people you plan to email. For your own setup, the free DKIM checker looks up a selector on your domain and shows whether the key is published and readable.
Authentication and list quality work together. A perfectly signed email sent to dead addresses still bounces, and a clean list sent without authentication still gets filtered.