How to set up DKIM on IONOS

Setting up DKIM on IONOS 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.

IONOS is the DNS host that publishes the DKIM record once your mail provider has generated it. Where IONOS also runs the mailboxes, it is the mail provider too and can toggle DKIM on without a manual record.

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.

Where IONOS Mail runs the mailboxes, IONOS offers a one-click DKIM toggle under the email settings and there is nothing to add by hand. Otherwise IONOS only publishes the TXT or CNAME record you enter, and the key itself comes from your mail provider.

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.

Sign in to IONOS, open Domains & SSL, select your domain, and check the Nameserver details. If they are IONOS nameservers, add the record at IONOS. If they point elsewhere, another host, Cloudflare or your email provider, add the DKIM record there instead.

Add the record in IONOS

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 IONOS expects it, the record looks like this:

TXT record from Google Workspace: v=DKIM1; k=rsa; p=<key provided by your mail provider>
CNAME records from Microsoft 365: selector1._domainkey and selector2._domainkey, each pointing at a target ending onmicrosoft.com

Type is TXT for most mail providers or CNAME for Microsoft 365. Host name takes the selector alone, for example google._domainkey or selector1._domainkey, because IONOS appends the domain itself. Value takes the long key beginning v=DKIM1; for a TXT record, or the target host ending onmicrosoft.com for a CNAME record. TTL stays on the default.

1. Sign in to IONOS and open the Domains & SSL area.
2. Select your domain, then open the DNS settings, labelled Adjust DNS settings or DNS.
3. Click Add record to open the blank record form.
4. Set Type to match what your provider gave you: TXT for most providers, or CNAME for Microsoft 365.
5. In the Host name field, enter only the selector part, for example google._domainkey or selector1._domainkey.
6. Do not add your domain name on the end, because IONOS appends it automatically.
7. In the Value field, paste the value your provider gave you: for a TXT record, the long key beginning v=DKIM1;, or for a CNAME record, the target host ending onmicrosoft.com.
8. Leave TTL on the default.
9. Click Save to publish the record.
10. For Microsoft 365, repeat the whole record for the second selector.

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 IONOS:

• Google Workspace supplies a TXT record and Microsoft 365 supplies two CNAME records. Adding the wrong type at IONOS means DKIM never validates.
• If your provider displays the value as google._domainkey.yourdomain.com, enter only google._domainkey in the Host name field. Including the domain again produces the broken host google._domainkey.yourdomain.com.yourdomain.com.
• DKIM public keys run to hundreds of characters. Check that nothing was cut off and that no stray spaces or line breaks were pasted in.
• Paste the plain value without adding your own quotation marks, because IONOS handles the quoting itself and manually added marks can corrupt the record.
• Some mail providers, Google Workspace included, require you to return to their console and start signing after the record is live. Publishing the record at IONOS is not enough on its own.

Save, then wait, then check

DNS changes at IONOS can take minutes up to a couple of hours before the provider can confirm the record and DKIM starts validating. The saved record appears in the DNS settings list under the domain in Domains & SSL.

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.

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.