DKIM, short for DomainKeys Identified Mail, is a digital signature added to your outgoing emails so receiving inboxes can confirm the message really came from your domain and was not changed in transit. The signature is checked against a public key you publish in your domain's DNS records.
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DomainKeys Identified Mail: a digital signature added to your emails so inboxes can confirm they really came from your domain and were not changed.
When you send an email, your mail server signs certain parts of the message with a private key. The receiving server looks up the matching public key in your DNS, a record usually set up by your email provider, and checks the signature. If it matches, the message passes DKIM.
DKIM works alongside SPF, which lists the servers allowed to send for your domain, and DMARC, which tells inboxes what to do when checks fail. Google's sender guidelines require bulk senders of more than 5,000 messages a day to Gmail to use SPF, DKIM and DMARC, and recommend a 2048 bit DKIM key where the provider supports it.
Every tool that sends on your behalf, such as an outreach platform, ATS or newsletter service, needs its own DKIM setup for your domain, or its mail may fail checks.
Why it matters to recruiters
Recruiting outreach lives or dies on inbox placement. Unsigned or failing mail is more likely to land in spam, which hurts reply rates and your domain's reputation. Agencies sending cold business development email and teams running candidate campaigns both need DKIM working on every sending domain and tool.
An example from a recruiting desk
A recruiting team connects a new sequencing tool to its outreach domain but skips the DKIM step. Reply rates fall by half in two weeks. A deliverability check shows the tool's mail fails DKIM. They add the provider's DKIM record to DNS, confirm it passes with a checker and see replies recover over the next month.
Tips that help
Set up DKIM for each service that sends mail from your domain, not only your main inbox.
Use a 2048 bit key if your email provider supports it.
Test with a free SPF, DKIM and DMARC checker after every DNS change.
Rotate keys when staff with DNS access leave or a provider changes.
Mistakes to avoid
Assuming DKIM is on by default when the provider requires you to add a DNS record.
Forgetting DKIM on a secondary sending domain used for cold outreach.
Frequently asked questions
Do I need DKIM if I already have SPF?
Yes. SPF checks which servers can send for your domain, while DKIM proves the message was not altered and is tied to your domain. Major inbox providers expect both, and bulk senders to Gmail are required to use both.
How do I check if DKIM is working?
Send a test email to a Gmail account and view the original message to see whether DKIM shows as pass. You can also use an online DNS checker to confirm the record exists and is valid.
Can DKIM stop my emails going to spam?
It helps but does not guarantee inbox placement. Content, sending volume, complaint rates and list quality all affect filtering. DKIM is a basic requirement, not a fix for poor outreach practice.
