What Is DMARC?
Email remains one of the most important communication channels for entreprises, but it is also one of the most commonly abused. Cybercriminals frequently attempt to impersonate legitimate domains through phishing, spoofing, and business email compromise attacks.
Domain-based Message Authentication, Rapports, and Conformance (DMARC) is an email authentication standard designed to protect domains from unauthorized email use. It works alongside two other email security technologies:
- Cadre de politique de l'expéditeur (FPS)
- Courrier identifié par DomainKeys (DKIM)
Ensemble, these technologies help receiving mail servers determine whether incoming messages are legitimate or potentially fraudulent.
DMARC provides instructions that tell receiving servers how to handle messages that fail authentication checks, helping domain owners reduce the risk of spoofed emails reaching users.
Why DMARC Matters
Without proper email authentication, attackers can forge your domain name and send messages that appear to originate from your organization.
Potential risks include:
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- Phishing attacks
- CEO fraud
- Credential theft
- Malware delivery
- Brand reputation damage
- Customer trust issues
DMARC helps reduce these threats by enforcing authentication policies and providing visibility into how your domain is being used across the internet.
How DMARC Works
DMARC relies on SPF and DKIM validation.
When an email is received, the receiving mail server performs several checks:
- Verify SPF authentication.
- Verify DKIM authentication.
- Check domain alignment requirements.
- Apply the DMARC policy.
- Generate reports if configured.
Depending on the configured policy, messages that fail authentication can be:
- Allowed
- Quarantined
- Rejected
This process helps prevent unauthorized senders from abusing your domain.
What Is a TXT Record?
DNS TXT records are special DNS entries that store text-based information.
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They are commonly used for:
- Domain verification
- SPF records
- DKIM records
- DMARC policies
- Third-party service validation
DMARC is implemented through a TXT record published in DNS.
Mail servers query this record to determine how authentication failures should be handled.
What Is a DMARC TXT Record?
A DMARC record is a specially formatted TXT record that contains policy instructions for email authentication.
The record is published under a dedicated DNS hostname:
_dmarc.yourdomain.com
When a receiving mail server processes a message claiming to originate from your domain, it retrieves this record and follows the specified policy.
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Example DMARC Record
A simple DMARC record may look like:
v=DMARC1; p=none; rua=mailto:[email protected]
This record contains several tags that define how DMARC operates.
Understanding DMARC Tags
v Tag
The version tag identifies the DMARC protocol version.
Exemple:
v=DMARC1
Actuellement, DMARC1 is the standard version used across the industry.
p Tag
The policy tag defines how failed messages should be handled.
Exemple:
p=none
This is the most important part of the record because it controls enforcement behavior.
rua Tag
The Reporting URI for Aggregate Reports.
Exemple:
rua=mailto:[email protected]
Aggregate reports provide visibility into authentication results and email activity associated with your domain.
These reports are typically delivered daily by participating mail providers.
DMARC Policy Types
The policy tag determines the action taken when a message fails authentication.
None Policy
p=none
Behavior:
- No enforcement action
- Messages continue to be delivered normally
- Reports are generated
This policy is commonly used during initial deployment because it allows administrators to monitor authentication results before enforcing stricter controls.
Quarantine Policy
p=quarantine
Behavior:
- Failed messages are treated as suspicious
- Messages may be delivered to spam or junk folders
This policy provides moderate protection while still allowing administrators to review potentially legitimate messages.
Reject Policy
p=reject
Behavior:
- Failed messages are rejected entirely
- Messages never reach the recipient mailbox
This is the strongest DMARC policy and offers the highest level of protection against spoofing attacks.
Understanding DMARC Reports
DMARC reporting provides valuable insight into how email is being sent from your domain.
Reports help identify:
- Unauthorized senders
- Misconfigured email services
- Spoofing attempts
- Échecs d'authentification
- SPF alignment issues
- DKIM alignment issues
Organizations often use these reports to fine-tune their email authentication strategy before moving to stricter policies.
Common DMARC Reporting Types
Aggregate Reports (RUA)
Aggregate reports provide summarized authentication data from participating mail providers.
These reports typically include:
- Sending IP addresses
- Authentication results
- Message volumes
- SPF status
- DKIM status
Forensic Reports (RUF)
Forensic reports provide detailed information about individual authentication failures.
Because of privacy considerations, support for forensic reporting varies between providers.
Creating a DMARC Record
Before publishing a DMARC policy, ensure that SPF and DKIM are already configured correctly.
Without SPF and DKIM alignment, DMARC enforcement may inadvertently block legitimate email.
Check Existing DNS Records
Before creating a new record, verify whether a DMARC record already exists.
Several DNS analysis tools can help verify current configuration.
Les options courantes incluent:
- MxToolbox DMARC Lookup
- DMARC analyzers
- DNS inspection tools
Enter your domain name and check whether a DMARC TXT record is already present.
Access DNS Management
To add or modify a DMARC record, access the DNS management interface used by your domain.
This may be located within:
- Domain registrar dashboards
- Hosting control panels
- DNS hosting providers
- CDN platforms
Once inside the DNS management area, you can create or edit records.
Méthode 1: Manually Create a DMARC Record
Étape 1
Create a new DNS record.
Étape 2
Sélectionner:
TXT
as the record type.
Étape 3
Set the hostname:
_dmarc
Étape 4
Add the DMARC value.
Exemple:
v=DMARC1; p=none; rua=mailto:[email protected]
Étape 5
Save the record.
DNS propagation may require some time before the record becomes visible globally.
Méthode 2: Use a DMARC Generator
Many DNS providers and email security platforms include DMARC generation tools.
These tools simplify configuration by allowing administrators to select options through a graphical interface.
Typical steps include:
- Open the email security section.
- Select DMARC configuration.
- Enter a reporting email address.
- Choose a policy.
- Define enforcement settings.
- Generate the record.
- Publish the generated TXT record.
This method reduces the chance of syntax errors.
Recommended Deployment Strategy
For new deployments, avoid immediately enforcing a reject policy.
A common rollout strategy is:
Phase 1
Monitor only:
p=none
Collect reports and verify all legitimate mail sources.
Phase 2
Increase protection:
p=quarantine
Monitor results and resolve any remaining authentication issues.
Phase 3
Full enforcement:
p=reject
Once all legitimate senders pass SPF and DKIM validation, move to full rejection.
This gradual approach minimizes the risk of accidentally blocking valid messages.
Best Practices for DMARC
Configure SPF and DKIM First
DMARC depends on proper SPF and DKIM alignment.
Verify both technologies before enabling enforcement.
Monitor Reports Regularly
Review aggregate reports to identify:
- Unauthorized senders
- Misconfigurations
- New email services
Use a Dedicated Reporting Address
Create a mailbox specifically for DMARC reports.
Exemple:
[email protected]
This helps separate reporting traffic from normal business communications.
Enforce Gradually
Move from:
none → quarantine → reject
rather than enabling rejection immediately.
Audit Third-Party Senders
Verify that all legitimate services sending email on behalf of your domain are properly authorized.
Les exemples incluent:
- Marketing platforms
- Help desk systems
- CRM software
- Transactional email services
Strengthening Email Security with DMARC
DMARC is one of the most effective tools available for protecting domains against spoofing, hameçonnage, and email impersonation attacks. By combining DMARC with SPF and DKIM, organizations gain greater control over how their domains are used and significantly improve email security.
A properly configured DMARC policy not only protects your organization but also improves email trustworthiness and increases the likelihood that legitimate messages reach their intended recipients.
