A email header helps you to detect phishing emails and botnet spam even before they are opened. By analyzing headers, you can reliably trace the sender, authentication check, delivery chain and manipulations.
Key points
- Header information provide technical details on the origin and delivery of each message.
- SPF, DKIM and DMARC values show whether a mail is legitimate or has been manipulated.
- IP addresses and Received lines help to find the origin server.
- Tools for header analysis facilitate the evaluation and visualization of key features.
- Spam indicators such as X-Spam status and BCL values indicate unwanted or dangerous content.
What is an email header?
In addition to the visible text, every email also contains an invisible section - the email header. This consists of technical information stored line by line. Among other things, it shows which server the message was sent via, when it was sent and how the recipient's mail server checked it. It also contains authentication results and security information.
A complete header usually begins with the first Received-line - this chronologically documents each node in the delivery chain. The earlier it appears, the closer it is to the original source. There is also control data such as Return path, Message ID, Authentication results or X-Spam status.
Header fields with security relevance
Some fields in an email header are particularly helpful if you want to determine the origin of a spam email. These include the complete Received chainbut also fields such as Authentication results and X-Headers.
SPF, DKIM and DMARC data are also transmitted in the header - e.g. like this:
| Field | Description | Example |
|---|---|---|
| Authentication results | Result of the domain authentication | spf=pass; dkim=pass; dmarc=fail |
| Received | Reception history through SMTP hops | from mail.example.com (IP) by mx.google.com |
| X-Spam status | Result of the spam filter check | YES, score=9.1 required=5.0 |
Identify spam sources based on the received lines
The Received lines provide information about which server stations a message was transported via. The last Received line stands for the original server - i.e. the real source. Spam senders often use manipulated headers or loops to disguise the path.
You should first look at the oldest received line and check whether it contains unusual IP addresses, server names or unusual time deviations. By looking at GeoIP mapping or DNS resolution, you can find out where this server is located and whether it belongs to a legitimate provider - or whether it is registered in known spam networks. In critical cases, a comparison with databases such as Spam House.
Recognize manipulated sender information
Spammers often manipulate the field of view address ("From:"), reply-to fields or return path. The sender is made to believe that the email comes from a trustworthy partner or customer. However, the headers reveal a lot about the technically responsible mail server - there are contradictions here.
If the "From:" and "Return-Path:" do not match, you should be suspicious. A Reply-To field that leads to a completely different domain also indicates attempts at deception. Some phishing emails even contain fake DKIM signatures or supposedly valid domain names - but the header shows the actual route through the network.
Read out Auth checks: SPF, DKIM and DMARC
To protect against spoofing and bot mail, most mail servers rely on authentication procedures. These generate machine-readable check results, for example:
- SPF: Was the sender authorized to send via this IP?
- DKIM: Does the crypto key match the sending domain?
- DMARC: Do From address and authentication go together?
You can find this information in the "Authentication-Results:" line. They look different depending on the provider, but often contain SPF=pass, dkim=fail or dmarc=pass. If, for example, DMARC fails but the mail appears to be correct, you should analyze the header further or perform an additional DNS check. In such cases, it makes sense to take a closer look at DMARC reports - supported by tools such as SecureNet for DMARC Reports.
Spam evaluation by filters: X-Spam status and BCL
Many emails contain additional fields that have been inserted by an internal spam filter. These fields contain a score or status indicating how likely the content is to be unwanted. These are particularly common:
X-Spam status: Shows whether the message exceeds the threshold of the internal filter (e.g. YES, score=9.3).
X-Spam-Level: Contains a number of asterisks ("*") that represent a threshold value.
BCL value: Microsoft-family mails contain Business Category Level - a risk factor between 0 and 9.
If these values are very high, you should actively start tracking and sender research. You can often find crucial information about the configuration and score using evaluation tools or by comparing with other headers from the same channel.
Analysis tools for header evaluation
Reviewing headers manually can be time-consuming. This is why many tools offer graphical analysis, display intermediate steps in color or allow direct IP checks. Popular solutions include
- Microsoft Message Header Analyzer (Office365 compatible)
- Google Header Analyzer (for Gmail)
- Mailheader.net (cross-platform, open source)
These tools explicitly highlight SPF, DKIM or DMARC evaluations. IP-DNS mappings, incorrect configurations or incomplete chains can also be quickly identified at a glance. I like to use them when evaluating forwarding or incoming bulk mails.
Log data as a supplementary source: mail server analysis
In addition to the email header, mail server logs also provide further information. Here you can easily identify correlating message flows, incorrect deliveries or recurring senders. Detailed logs are particularly helpful in enterprise environments with Postfix, Exim or Microsoft Exchange.
The combination of headers and logs gives a more complete picture. For example, I look for suspicious message ID chains or IP domains that repeatedly deliver incorrect SPF data. The technical evaluation can be designed efficiently - for example with Postfix log file analyses and automated bounces.
Classification of legitimate transport routes
Not every unusual-looking header line is automatically suspicious. A third-party service may have been involved: Newsletter services, CRM systems or internal gateways often change DKIM/SPF entries. Context is crucial here.
You should regularly save reference headers from legitimate senders. This will allow you to immediately recognize the difference in unusual cases. This increases the speed of routing diagnostics or critical delivery errors.
Reliably detect sources of spam through header analysis
Email headers contain numerous technical clues that allow you to detect misuse and unwanted content in a much more targeted manner. You can uncover hidden server chains, authentication errors or targeted forgeries. This is much more effective than checking content alone.
By regularly checking suspicious headers and documenting anomalies, you can improve your delivery rates and secure your mail routing. You can also protect your infrastructure from list entries and abuse escalations.
Advanced procedures for complex cases
Particularly in larger corporate environments or with intensive email traffic, it is often the case that several forwarding and intermediate stations appear in the received lines. For example, companies send via external mailing list providers or use central anti-spam appliances. This results in additional Received and X header fields, which can be confusing at first glance. In such situations, it can be helpful to draw detailed diagrams or generate an automated list using header analysis tools.
If you often have to deal with complex headers, you can also create a checklist. This contains the typical elements and their expected content. In the list, indicate which IPs are stored as authorized source servers in your SPF zone and which DKIM selectors should appear in emails. As soon as you find deviations, this is a good indicator of a possible manipulation of the header.
- Comparison with reference headers: Have examples of legitimate emails from your domain ready.
- Regular maintenance of your DNS records: Changed IP structures should always be reflected promptly in SPF and DMARC.
- Consideration of forwarders: Check whether ARC (Authenticated Received Chain) is used to pass on authentication information.
Message ID and its meaning
The field is also revealing Message ID. Each email sent is assigned a unique identifier when it is created or transported. However, some spam campaigns use generic or completely random message IDs that cannot be matched to the sender or the content. Duplicate message IDs or conspicuously simple IDs can also indicate automated spam tools.
In some cases, you can use log files or archive systems to find similar message IDs and thus recognize patterns. This allows you to detect serial phishing campaigns more quickly. If the message ID is also inconsistent with the domain in the "From:" field, this increases the likelihood that sender information has been falsified here.
Supplementary safety standards: ARC and BIMI
The Authenticated Received Chain (ARC) can be used for complex mail flows with forwarding or mailing lists in particular. It makes it possible to pass on authentication results across intermediate stations so that the recipient mail servers can better assess whether a mail is legitimate. You can recognize this in the header by lines such as ARC-Seal or ARC-Authentication-Results. If these headers are managed correctly, an original DKIM signature remains valid even after forwarding.
There are also newer initiatives such as BIMI (Brand Indicators for Message Identification), which can display the sender's logo if DMARC is implemented correctly. Although BIMI is not a direct component of the email header in terms of the delivery chain, it only works reliably if the header data such as DKIM and DMARC can be evaluated correctly. In this way, BIMI provides a visual indication of whether an email actually originates from the brand owner or whether it is a forgery.
Typical misconfigurations and how to find them
Not all conspicuous headers are the result of malicious intent. Simple configuration errors are often behind them. For example, your own mail server may inadvertently deliver incorrect SPF or DKIM entries if you did not adjust the DNS correctly when changing hosters. Entries with "softfail" instead of "pass" also sometimes indicate that the sender IP is not included in the SPF record.
- Missing DKIM signature: Signing services accidentally not activated or incorrectly configured.
- Inappropriate IPs in the SPF entry: New or deleted IPs not updated.
- Forgotten subdomains: Subdomains are easily overlooked, especially in larger organizations, so that no correct SPF record is entered there.
If you keep coming across the same misconfiguration during the header check, you should verify the sender's DNS entry and correct any typos. This will reduce the false positives rate for legitimate emails and at the same time control effective spam defense.
Monitoring and response times
An effective anti-spam strategy requires you to be able to recognize conspicuous emails quickly and react accordingly. Depending on how large your network is or how many emails you receive every day, automation may be worthwhile. Many companies set up SIEM or log management systems that automatically scan email headers and check for threats. Certain threshold values are defined above which an incident is reported.
Another useful measure is the regular training of employees who work in customer support or in the IT team. They should know how to immediately recognize the worst spam indicators in the header. In this way, you can prevent a critical email from remaining in the system for too long before it is identified as a threat. Once an incident has been identified, a clear incident response routine supports further investigation. This is where the tools and techniques described in the article come into play again.
Integration of header analysis into the overall security process
A thorough header analysis should never be carried out in isolation. You can combine it with other security measures to create a holistic defense chain. For example, after finding a suspicious IP address, you not only check the SPF status, but also carry out an antivirus and link analysis in the message body. True botnet spam waves are often based on multiple attack angles, including manipulated attachments, forged links or Trojans.
If you operate a uniform security stack, you can also combine the results from the header analysis with information from firewalls, endpoint security or web server logs. An IP that is currently causing failed logins or DDoS attacks for several services is particularly suspicious if it appears in email received lines at the same time. This allows you to achieve a significantly faster response time and a comprehensive security assessment.
Best practices for efficient header evaluation
In order to be successful in the long term, it is advisable to adhere to a few basic principles when evaluating headers. These include
- Proceed systematically: Work according to a defined sequence, e.g. Received lines first, then authentication results, then X headers.
- Update regularly: Keep knowledge bases and reference headers for your most important communication partners up to date.
- Use automated tools: Some things can be done faster and more securely using specialized analysis tools - especially in large-volume environments.
- Sustainable documentation: Any anomaly in the header can be part of a larger pattern. Use internal tickets or logs to manage incidents in a traceable manner.
In teams in particular, it is a good idea to keep a knowledge database and collect specific email examples - including headers, authentication results and a brief summary of the analysis. This way, even new colleagues can quickly learn what is important when assessing a suspicious email.
Shared benefits for sender and recipient
A clean and traceable email infrastructure is not only important for recipients, but also for you as the sender. Anyone who sends professional newsletters or transactional emails must ensure that all authentication mechanisms are configured correctly. Otherwise emails may end up in the spam folder unintentionally. A correct SPF entry, valid DKIM signatures and a suitable DMARC policy ensure that reputable senders are immediately recognizable and less likely to get caught in questionable filters.
Recipients in turn benefit from clearly visible routes and authentication results, as they can detect potential attacks or forgery attempts much more quickly. This results in a transparent email exchange on both sides, which creates trust and minimizes attack surfaces.
Summary
Header analysis is one of the most important techniques for checking email traffic and detecting spam or phishing attacks before they cause any damage. By looking at received chains, authentication checks (SPF, DKIM, DMARC) and specifically inserted fields such as X-Spam-Status or BCL values, you can uncover hidden tricks and targeted attempts at deception. In complex environments, it helps to proceed systematically, maintain reference headers and document deviations precisely. At the same time, it is worth combining tools and log analyses to obtain reliable results.
Those who regularly analyze email headers and deal with the patterns they uncover not only benefit from greater security and lower spam rates, but also improve their own delivery rate. A structured approach, the right tools and a solid foundation of knowledge about DNS records, mail server behavior and fiasco-prone configurations are the keys to fail-safe and reliable mail traffic.
