Use DKIM to validate outbound email sent from your custom domain in Office 365

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Topic Last Modified: 2017-06-19

Summary: This article describes how you use DomainKeys Identified Mail (DKIM) with Office 365 to ensure that destination email systems trust messages sent from your custom domain.

You should use DKIM in addition to SPF and DMARC to help prevent spoofers from sending messages that look like they are coming from your domain. DKIM lets you add a digital signature to email messages in the message header. Sounds complicated, but it's really not. When you configure DKIM, you authorize your domain to associate, or sign, its name to an email message by using cryptographic authentication. Email systems that receive email from your domain can use this digital signature to help determine if incoming email that they receive is legitimate.

Basically, you use a private key to encrypt the header in your domain's outgoing email. You publish a public key to your domain's DNS records that receiving servers can then use to decode the signature. They use the public key to verify that the messages are really coming from you and not coming from someone spoofing your domain.

Office 365 automatically sets up DKIM for initial domains. The initial domain is the domain that Office 365 created for you when you signed up with the service, for example, contoso.onmicrosoft.com. You don't need to do anything to set up DKIM for your initial domain. For more information about domains, see Domains FAQ.

You can choose to do nothing about DKIM for your custom domain too. If you do not set up DKIM for your custom domain, Office 365 creates a private and public key pair, enables DKIM signing, and then configures the Office 365 default policy for your custom domain. While this is sufficient coverage for most Office 365 customers, you should manually configure DKIM for your custom domain in the following circumstances:

  • You have more than one custom domain in Office 365

  • You're going to set up DMARC too (recommended)

  • You want control over your private key

  • You want to customize your CNAME records

  • You want to set up DKIM keys for email originating out of a third-party domain, for example, if you use a third-party bulk mailer.

In this article:

SPF adds information to a message envelope but DKIM actually encrypts a signature within the message header. When you forward a message, portions of that message's envelope can be stripped away by the forwarding server. Since the digital signature stays with the email message because it's part of the email header, DKIM works even when a message has been forwarded as shown in the following example.

Diagram showing a forwarded message passing DKIM authentication where the SPF check fails

In this example, if you had only published an SPF TXT record for your domain, the recipient's mail server could have marked your email as spam and generated a false positive result. The addition of DKIM in this scenario reduces false positive spam reporting. Because DKIM relies on public key cryptography to authenticate and not just IP addresses, DKIM is considered a much stronger form of authentication than SPF. We recommend using both SPF and DKIM, as well as DMARC in your deployment.

The nitty gritty: DKIM uses a private key to insert an encrypted signature into the message headers. The signing domain, or outbound domain, is inserted as the value of the d= field in the header. The verifying domain, or recipient's domain, then use the d= field to look up the public key from DNS and authenticate the message. If the message is verified, the DKIM check passes.

To configure DKIM, you will complete these steps:

For each domain for which you want to add a DKIM signature in DNS, you need to publish two CNAME records. A CNAME record is used by DNS to specify that the canonical name of a domain is an alias for another domain name.

Office 365 performs automatic key rotation using the two records that you establish. If you have provisioned custom domains in addition to the initial domain in Office 365, you must publish two CNAME records for each additional domain. So, if you have two domains, you must publish two additional CNAME records, and so on.

Use the following format for the CNAME records:

Host name:			selector1._domainkey.<domain>
Points to address or value:	selector1-<domainGUID>._domainkey.<initialDomain> 
TTL:				3600

Host name:			selector2._domainkey.<domain>
Points to address or value:	selector2-<domainGUID>._domainkey.<initialDomain> 
TTL:				3600

Where:

  • For Office 365, the selectors will always be "selector1" or "selector2".

  • domainGUID is the same as the domainGUID in the customized MX record for your custom domain that appears before mail.protection.outlook.com. For example, in the following MX record for the domain contoso.com, the domainGUID is contoso-com:

    contoso.com.  3600  IN  MX   5 contoso-com.mail.protection.outlook.com
    
  • initialDomain is the domain that you used when you signed up for Office 365. For information about determining your initial domain, see Domains FAQ.

For example, if you have an initial domain of cohovineyardandwinery.onmicrosoft.com, and two custom domains cohovineyard.com and cohowinery.com, you would need to set up two CNAME records for each additional domain, for a total of four CNAME records.

Host name:			selector1._domainkey.cohovineyard.com  
Points to address or value:	selector1-cohovineyard-com._domainkey.cohovineyardandwinery.onmicrosoft.com
TTL:				3600

Host name:			selector2._domainkey.cohovineyard.com  
Points to address or value:	selector2-cohovineyard-com._domainkey.cohovineyardandwinery.onmicrosoft.com
TTL:				3600

Host name:			selector1._domainkey.cohowinery.com
Points to address or value:	selector1-cohowinery-com._domainkey.cohovineyardandwinery.onmicrosoft.com 
TTL:				3600
 
Host name:			selector2._domainkey.cohowinery.com
Points to address or value:	selector2-cohowinery-com._domainkey.cohovineyardandwinery.onmicrosoft.com 
TTL:				3600

Once you have published the CNAME records in DNS, you are ready to enable DKIM signing through Office 365. You can do this either through the Office 365 admin center or by using PowerShell.

  1. Sign in to Office 365 with your work or school account.

  2. Select the app launcher icon in the upper-left and choose Admin.

  3. In the lower-left navigation, expand Admin and choose Exchange.

  4. Go to Protection > dkim.

  5. Select the domain for which you want to enable DKIM and then, for Sign messages for this domain with DKIM signatures, choose Enable. Repeat this step for each custom domain.

  1. Connect to Exchange Online using remote PowerShell.

  2. Run the following cmdlet:

    New-DkimSigningConfig -DomainName <domain> -Enabled $true
    

    Where domain is the name of the custom domain for which you want to enable DKIM signing.

    For example, for the domain contoso.com:

    New-DkimSigningConfig -DomainName contoso.com -Enabled $true
    

Wait a few minutes before you follow these steps to confirm that you have properly configured DKIM. This allows time for the DKIM information about the domain to be spread throughout the network.

  • Send a message from an account within your Office 365 DKIM-enabled domain to another email account such as outlook.com or Hotmail.com.

  • Do not use an aol.com account for testing purposes. AOL may skip the DKIM check if the SPF check passes. This will nullify your test.

  • Open the message and look at the header. Instructions for viewing the header for the message will vary depending on your messaging client. For instructions on viewing message headers in Outlook, see View e-mail message headers.

    The DKIM-signed message will contain the host name and domain you defined when you published the CNAME entries. The message will look something like this example:

    From: Example User <example@contoso.com> 
    DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; 
        s=selector1; d=contoso.com; t=1429912795; 
        h=From:To:Message-ID:Subject:MIME-Version:Content-Type; 
        bh=<body hash>; 
        b=<signed field>;
    
    
  • Look for the Authentication-Results header. While each receiving service uses a slightly different format to stamp the incoming mail, the result should include something like DKIM=pass or DKIM=OK.

If at some point in the future you decide to add another custom domain and you want to enable DKIM for the new domain, you must complete the steps in this article for each domain. Specifically, complete all steps in What you need to do to manually set up DKIM in Office 365.

Disabling the signing policy does not completely disable DKIM. After a period of time, Office 365 will automatically apply the default Office 365 policy for your domain. For more information, see Default behavior for Exchange Online Protection (EOP) and Office 365.

To disable the DKIM signing policy by using Windows PowerShell
  1. Connect to Exchange Online using remote PowerShell.

  2. Run one of the following commands for each domain for which you want to disable DKIM signing.

    $p=Get-DkimSigningConfig -identity <domain>
    $p[0] | set-DkimSigningConfig -enabled $false
    
    

    For example:

    $p=Get-DkimSigningConfig -identity contoso.com
    $p[0] | set-DkimSigningConfig -enabled $false
    

    Or

    Set-DkimSigningConfig -identity $p[<number>].identity -enabled $false
    
    

    Where number is the index of the policy. For example:

    Set-DkimSigningConfig -identity $p[0].identity -enabled $false
    

If you do not enable DKIM, Office 365 automatically creates a 1024-bit DKIM public key for your custom domain and the associated private key which we store internally in our datacenter. By default, Office 365 uses a default signing configuration for domains that do not have a policy in place. This means that if you do not set up DKIM yourself, Office 365 will use its default policy and keys it creates in order to enable DKIM for your domain.

Also, if you disable DKIM signing after enabling it, after a period of time, Office 365 will automatically apply the Office 365 default policy for your domain.

In the following example, suppose that DKIM for fabrikam.com was enabled by Office 365, not by the administrator of the domain. This means that the required CNAMEs do not exist in DNS. DKIM signatures for email from this domain will look something like this:

From: Second Example <second.example@fabrikam.com> 
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; 
    s=selector1-fabrikam-com; d=contoso.onmicrosoft.com; t=1429912795; 
    h=From:To:Message-ID:Subject:MIME-Version:Content-Type; 
    bh=<body hash>; 
    b=<signed field>;

In this example, the host name and domain contain the values to which the CNAME would point if DKIM-signing for fabrikam.com had been enabled by the domain administrator. Eventually, every single message sent from Office 365 will be DKIM-signed. If you enable DKIM yourself, the domain will be the same as the domain in the From: address, in this case fabrikam.com. If you don’t, it will not align and instead will use your organization's initial domain. For information about determining your initial domain, see Domains FAQ.

Some bulk email service providers, or software-as-a-service providers, let you set up DKIM keys for email that originates from their service. This requires coordination between yourself and the third-party in order to set up the necessary DNS records. No two organizations do it exactly the same way. Instead, the process depends entirely on the organization.

An example message showing a properly configured DKIM for contoso.com and bulkemailprovider.com might look like this:

Return-Path: <communication@bulkemailprovider.com>
 From: <sender@contoso.com>
 DKIM-Signature: s=s1024; d=contoso.com
 Subject: Here is a message from Bulk Email Provider's infrastructure, but with a DKIM signature authorized by contoso.com

In this example, in order to achieve this result:

  1. Bulk Email Provider gave Contoso a public DKIM key.

  2. Contoso published the DKIM key to its DNS record.

  3. When sending email, Bulk Email Provider signs the key with the corresponding private key. By doing so, Bulk Email Provider attached the DKIM signature to the message header.

  4. Receiving email systems perform a DKIM check by authenticating the DKIM-Signature d=<domain> value against the domain in the From: (5322.From) address of the message. In this example, the values match:

    sender@contoso.com

    d=contoso.com

Although DKIM is designed to help prevent spoofing, DKIM works better with SPF and DMARC. Once you have set up DKIM, if you have not already set up SPF you should do so. For a quick introduction to SPF and to get it configured quickly, see Set up SPF in Office 365 to help prevent spoofing. For a more in-depth understanding of how Office 365 uses SPF, or for troubleshooting or non-standard deployments such as hybrid deployments, start with How Office 365 uses Sender Policy Framework (SPF) to prevent spoofing. Next, see Use DMARC to validate email in Office 365. Anti-spam message headers includes the syntax and header fields used by Office 365 for DKIM checks.

 
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