How to set up DKIM on Microsoft 365

Setting up DKIM on Microsoft 365 comes down to two moves: get the selector and the public key from whoever runs your mailboxes, then publish that key as a DNS record wherever your domain’s DNS is answered. The record comes first on this page because a person who can read one can tell a good paste from a broken one.

Microsoft 365 is the mailbox provider here, and it is also the party that generates the DKIM record values for your tenant. It does not host your domain’s DNS unless you have separately chosen Microsoft to manage it, so publishing the records happens somewhere else.

What DKIM proves, and where the key has to live

DKIM (DomainKeys Identified Mail) puts a signature on each message sent from your domain. The mail platform signs with a private key it keeps to itself. The receiving server pulls the selector out of the DKIM-Signature header, looks up <selector>._domainkey.yourdomain.com in DNS, and checks the signature against the public key it finds there. A match tells the receiver two things: the message came from a system holding your private key, and nobody changed the body or the signed headers on the way.

That lookup is why the record goes where your DNS lives. Receivers don’t ask your mail platform for the key, and they don’t ask your registrar unless it happens to run the zone. They ask whichever nameservers your domain delegates to, and only the copy held there counts. The DKIM guide covers the check from the receiver’s side; this page covers the publishing side.

Two parties share the record and neither can do the other’s part. The mail platform generates the key pair inside its own console, keeps the private half, signs outgoing mail with it, and hands you the public half plus a selector. The DNS host publishes what you paste and has no way to invent a value that would verify.

Microsoft generates the two CNAME target values for your tenant and holds the DKIM keys behind them, which is why it hands you two short pointers rather than one long key to copy and store yourself. Microsoft does not create the CNAME records in your DNS and does not sign any message until you switch the toggle on in the portal.

The record, tag by tag

Most platforms hand you a TXT record. Its value is a short run of tags separated by semicolons, and a typical one begins v=DKIM1; k=rsa; p= followed by the key. v=DKIM1 names the version and, when present, has to come first. k=rsa names the key type; k=ed25519 exists, and a platform that supports it will say so. p= carries the public key itself, hundreds of characters of base64 with no spaces, and it’s the one tag a receiver insists on.

Three optional tags matter. t=y marks the key as testing, so receivers treat a failed signature as if the message carried none. t=s says the identity in the signature has to be your domain itself, with no subdomain in front of it. h=sha256 limits the hash to the algorithm receivers use anyway.

Start with the value as your platform issued it, character for character, and don’t tidy it. Once mail is flowing, tighten in two places. If t=y is present, remove it so a failure counts as a failure. If the platform offers a 2048-bit key alongside a 1024-bit one, pick 2048, since it’s the length platforms now issue by default and receivers accept it.

Rotation stays the platform’s job: when it issues a new key under a new selector, you publish the new record and leave the old one up until nothing signs with it any more.

Some platforms give you a CNAME instead. The name is still <selector>._domainkey, and the target is a host on the platform’s own servers that holds the TXT value, which lets the platform rotate keys without you touching DNS again. If you were given a CNAME, publish a CNAME. Converting the target into a TXT record publishes a hostname where receivers expect a key.

The selector is the half people underrate. A receiver builds its lookup from the s= tag in the header, so the name you publish has to match the one the platform signs with, letter for letter. selector1 and selector2 are different keys, and so are mail and mail1. For DMARC alignment the d= in the signature also has to be your domain or a subdomain of it.

Confirm who answers DNS for your domain

A record saved in a console that your domain doesn’t delegate to is invisible to receivers, whatever that console reports.

Microsoft 365 is not necessarily your DNS host, so check the Nameservers section of your registrar account to see which company answers DNS for the domain. The two CNAME values shown on the DKIM page have to be published at that company, not inside the Microsoft security portal. If the nameservers point to Microsoft, publish the records there instead.

Add the record in Microsoft 365

Copy both pieces from the platform’s DKIM or email authentication page and keep that tab open until the check at the end passes. Laid out the way Microsoft 365 expects it, the record looks like this:

selector1._domainkey CNAME selector1-<your-domain>._domainkey.<your-tenant>.onmicrosoft.com
selector2._domainkey CNAME selector2-<your-domain>._domainkey.<your-tenant>.onmicrosoft.com

Both records use type CNAME, never TXT. The Name or Host field takes only selector1._domainkey or selector2._domainkey, since the DNS host appends the domain itself; typing the domain a second time produces a broken host such as selector1._domainkey.yourdomain.com.yourdomain.com. The Value, Points to or Target field takes Microsoft’s target exactly as shown on the DKIM page, with no surrounding quote marks. Leave TTL on the default and save both records.

1. Sign in as an administrator and open the Microsoft security portal at security.microsoft.com.
2. Go to the Email & collaboration area in the left-hand menu.
3. Open Policies & rules, then Threat policies, then Email authentication settings, then DKIM.
4. Select your domain from the list shown on the DKIM page.
5. Read off Host 1, selector1._domainkey, and its target, selector1-<your-domain>._domainkey.<your-tenant>.onmicrosoft.com.
6. Read off Host 2, selector2._domainkey, and its target, selector2-<your-domain>._domainkey.<your-tenant>.onmicrosoft.com.
7. Publish both CNAME records at your DNS host, then return to this DKIM page.
8. Switch the control labelled Sign messages for this domain with DKIM signatures to On, shown in some tenants as Enable.

Before you save, compare the last few characters of the pasted value with the platform’s page, because a copy that stops early publishes a key that will never verify, and the console won’t warn you. Check the selector in the name against the platform’s page in the same pass.

A few traps are specific to Microsoft 365:

• Microsoft’s DKIM record is a CNAME, not a TXT public key, so pasting a TXT value the way another provider’s guide describes will not work here.
• Publishing both CNAMEs is not the same as switching signing on. Microsoft signs nothing until you return to the DKIM page and turn the toggle to On.
• The onmicrosoft.com target carries your tenant name and unique codes, so one wrong character stops DKIM validating.
• A CNAME target is a plain host name and takes no quote marks, unlike a TXT record.
• Microsoft checks that both records are visible in DNS before it lets you switch signing on, so the toggle can fail if you attempt it too soon after publishing.

Save, then wait, then check

DNS changes take minutes to a couple of hours before Microsoft can confirm the two records are visible. The records you save appear on the same DKIM page in the Microsoft security portal, against your domain.

Once the record shows in the console, run the free scan against your domain. It asks DNS for the selector the way a receiving server does and tells you in plain words whether the key is published and readable.

Microsoft checks that both CNAMEs resolve before it allows the toggle to switch to On, so a failed attempt there is a sign the records are not visible yet.

A second check closes the loop. Send a message from the domain to a mailbox you control and open the Authentication-Results header. dkim=pass with your domain in header.d= means the signature verified against the key you published. dkim=fail or dkim=none with the record in place points at the selector or at the platform side, and the next section says where to look.

When the key is missing or malformed

Without a record at the selector, the receiver gets no key and the message carries no verified signature. It won’t bounce on that alone, though DMARC then has only SPF to lean on, and SPF breaks on forwarding, so the quote a customer forwards to a colleague arrives with neither.

A malformed record fails in the same quiet way: a p= value cut short, a stray space or line break in the middle, a doubled host such as mail._domainkey.yourdomain.com.yourdomain.com, or a selector that’s off by one character, and the receiver logs dkim=fail or dkim=permerror and moves on. You hear nothing unless DMARC reports are arriving, and the DMARC piece shows how those reports expose a DKIM failure per sending source.

The census shows how ordinary that silence is. Of the 75,823,110 domains where the September 2026 census could run the DKIM check, it found a key at a known selector on 37,999,462 (50.1%). That check works the way any checker does: there’s no way to list the DKIM records on a domain, so it can only ask for selectors it knows to try, and a platform with an unusual selector can read as absent when the key is fine.

If your scan reports nothing found while the record sits in the console, finding your DKIM selector shows how to read the live selector out of a sent message’s headers before you change anything.

Work a failure in order: the selector spelling first, the nameservers second, then the tail of the pasted value, then whether the platform still needs you to switch signing on.

Figures as of 5 September 2026, from the September 2026 edition of the defaults.exposed census. Census numbers move every month; the current values are on the census data page.