Optimizing delivery: An overview of authentication methods

When sending emails today, it is no longer sufficient to simply transmit messages via a mail server. Internet service providers and email services such as Gmail, Outlook, and Yahoo increasingly verify whether the actual sender of an email is trustworthy.

Authentication methods help confirm the sender's identity and prevent abuse through spam, phishing, or spoofed emails. At the same time, they improve email deliverability and increase the likelihood that messages land in the inbox rather than the spam folder.

For businesses, newsletter senders, and organizations, these methods are now an essential component of professional email communication.

What is email authentication?

Email authentication refers to technical methods that allow receiving mail servers to verify whether an email actually originated from the stated sender.

This process employs various security mechanisms that provide information about the sending server, the domain used, and the integrity of the message.

The three most important standards are:

• SPF (Sender Policy Framework)
• DKIM (DomainKeys Identified Mail)
• DMARC (Domain-based Message Authentication, Reporting and Conformance)
  

These methods complement each other and should ideally be used in combination.

SPF – Sender Policy Framework

SPF is a method that allows domain owners to specify which mail servers are authorized to send emails on behalf of their domain.

To achieve this, a specific SPF record is added to the domain's DNS settings. This enables recipients to verify whether the email actually originated from an authorized server.

Benefits of SPF:

• Protection against sender spoofing
• Improved deliverability
• Support for spam detection
• Easy implementation
  

A correctly configured SPF record is often the first step toward improving email reputation.

DKIM – DomainKeys Identified Mail

DKIM complements SPF by adding a digital signature to every sent email. During transmission, a cryptographic signature is generated and embedded in the message header. The receiving mail server can verify this signature using a public key published in the DNS.

This ensures that:

• The email actually originates from the stated sender.
• The content has not been altered during transmission.
• The message is authentic and intact.
  

DKIM increases trust in the email and is considered by many major email providers to be a key requirement for good deliverability.

DMARC – Domain-based policies

DMARC builds upon SPF and DKIM and defines rules for handling emails that fail authentication checks.

The domain owner determines whether messages that fail validation should be:

• Allowed
• Quarantined
• Completely rejected
  

Additionally, reports regarding failed delivery attempts can be sent to designated email addresses.

Benefits of DMARC:

• Protection against phishing attacks
• Protection of brand identity
• Improved domain reputation
• Transparency through delivery reports
  

DMARC is increasingly regarded as the standard for professional senders.

The Interplay of SPF, DKIM, and DMARC

None of these mechanisms should be viewed in isolation. Maximum security and optimal deliverability are achieved when all three technologies are used together.

A typical verification process looks like this:

• The recipient checks SPF.
• The recipient checks DKIM.
• DMARC evaluates the results.
• Based on the defined policy, the message is accepted, flagged, or rejected.
  

This interaction enables much more reliable identification of legitimate email senders.

What is BIMI?

BIMI stands for Brand Indicators for Message Identification and represents a further development in the field of email authentication.

With BIMI, companies can display their brand logo next to authenticated emails in supporting inboxes.

Prerequisites generally include:

• Correctly configured SPF records
• Correctly configured DKIM signatures
• An active DMARC policy
  

BIMI enhances brand visibility and boosts recipient trust.

Impact on Deliverability

Missing or incorrect authentication can significantly affect deliverability. Many email providers now view such messages critically.

Possible consequences include:

• Delivery to the spam folder
• Warning notices displayed to the recipient
• Rejection of the email
• Lower sender reputation
• Restrictions on high-volume sending
  

Conversely, a correctly authenticated domain is often classified as more trustworthy.

Common Causes of Authentication Issues

Problems often arise from incorrect DNS records or incomplete configurations.

Typical causes include:

• Missing SPF records
• Multiple SPF records for a single domain
• Incorrect DKIM keys
• Inactive DKIM signatures
• Missing DMARC policies
• Changes to sending infrastructure without corresponding DNS updates
  

Therefore, authentication settings should be regularly reviewed and updated.

Best Practices for Optimal Deliverability

To achieve the highest possible delivery rate, the following measures should be implemented:

• Configure SPF for all sending servers.
• Enable DKIM signing.
• Set up a DMARC policy.
• Regularly analyze delivery reports.
• Use only trusted sending servers.
• Monitor domain reputation.
• Regularly check authentication settings.
  

These measures help strengthen the domain's trustworthiness over the long term.