Improve email security in Microsoft 365: Fine-tuning DKIM and setup DMARC for the MOERA domain.

Fine-tune DKIM by frequently rotating the DKIM keys

After setting up DKIM in Microsoft Defender for Office 365, it is also important to set up frequent rotation of these DKIM keys to prevent adversaries from intercepting and decrypting your cryptographic keys. Key rotation helps to minimize the risk of compromising the private keys. In Microsoft 365, you can rotate the DKIM keys for your domains to increase security. The recurrence must be every 3 months because rotating the DKIM keys every 3 months ensures a complete rotation of both selectors every 6 months. You can rotate the DKIM keys manually using the Defender portal or Exchange Online PowerShell, but it is easy to forget if you do it manually. So you should delegate this to Azure Automation by using the runbook below:

#Connect EXO
Connect-ExchangeOnline -ManagedIdentity -Organization

#Rotate the DKIM keys for all enabled domains
$Domains = (Get-DkimSigningConfig | Where-Object {$_.Enabled -eq 'True'}).domain

foreach ($domain in $domains){
    Rotate-DkimSigningConfig -KeySize 2048 -Identity $domain

To configure the runbook, you can access my MDO Azure Automation repository on Github.

Setup DMARC for the MOERA domain

Your domain, also known as the Microsoft Online Email Routing Address (MOERA) domain, does not have a DMARC policy set up by default. This means that this domain is vulnerable to abuse and can be used to spoof your domain. Spoofed inbound messages from your domain will end up in the Junk folder, because the detection technology is Spoof intra-org. Spoof Intelligence will not trigger your anti-phishing policy for this detection.

To prevent email spoofing and phishing using this domain, you need to set up a DMARC reject policy on the MOERA domain, as you do on all of your send and non-send domains. Setting DMARC for the MOERA is most often forgotten because you do not own the MOERA domain, but Microsoft has made it possible to manage the public DNS of your MOERA domain in the Microsoft 365 Admin Center.

  1. In the Microsoft 365 admin center, select Show all > Settings > Domains
  2. On the Domains page, open the * domain and click on DNS Records
  3. Click on Add record and set the following values
  • Type: TXT
  • TXT name: _dmarc
  • TXT value: v=DMARC1; p=reject;

To summerize

DKIM rotation ensures that even if a key is compromised, it will become obsolete after a period of time, limiting the window of vulnerability. Additionally, setting up DMARC for the MOERA domain is crucial to protect against email fraud targeting this domain.