Understanding SMTP Logs for Cyber Security Investigations

Consider email to be the holy grail of digital detectives. It carries along with it message text, as well as a pile of hidden clues, which makes it a hot spot to anybody tracking down shady activity. Understanding SMTP Logs plays a crucial role in uncovering these hidden file details. The SMTP log (Simple Mail Transfer Protocol log) is one of the most convenient tools in that toolkit. When applied correctly, it lets the cat out of the bag about phishing attacks, as well as the true author of that email. The SMTP log is a resource of choice when a person is tracking an email thread, checking the path, or drawing a timeline.

What is SMTP log analysis then, and why is it important? Imagine this: when your email program passes a message to a mail server, that server maintains some record, an SMTP log. Such records do not only indicate the name of the sender and recipient; they also indicate timestamps, IP addresses, and other nitty-gritty information. Investigators troll through such logs in order to determine how emails bounced between servers and who was in charge at each stage.

In short: when you are looking through the emails, in any way, the SMTP log is worth a fair perusal.

Read Next: Steps for Phishing Email Analysis

What Are SMTP Logs?

Just imagine SMTP logs as the receipts when every mail leaves your mail server. They are literally the notes your server makes in the sending and relaying of emails through the Internet.

The majority of log entries that you will encounter resemble:

• The IP address of sender
• The email address of the recipient
• The date of the transaction Date at which the transaction is entered.
• SMTP status codes (example, 250 OK or 550 Error)
• Message ID
• Server’s responses
• Authentication status

The layout of the various Mail Transfer Agents (MTAs), such as Microsoft Exchange, Postfix, Sendmail, and others, is slightly different, yet they all bear the common basic configuration.

Why Is SMTP Log Analysis Important?

Consider SMTP logs to be the email analog of a transaction log: each and every email that is sent or pulled off the server is logged. To the forensic analysts, they are gold. Understanding SMTP Logs can help resolve any dispute, reveal spammers, trace the delivery trail, and even inform you that an individual pressed the send button or merely left the humiliating email in their sent folder.

Anything you can paw at once:

1. Follow the trail of the malicious emails on their targets
2. Set timestamps and delivery date to be able to see whether a person fiddled with the log or not
3. Detect the rogue or hacked login or activity on the accounts
4. Find out the existence of a bulk or spam campaign
5. Focus on any goal to steal information on emails and seek the evidence

It would look like this, an employee would say that he/she did not send a mad message, SMTP would inform you whether an email left the server of the company, which IP it had been sent on, and whether it was signed. Boom, the problem is solved.

Understand SMTP Log Example

Here’s an example of SMTP log snippet:

Jul 8 10:23:14 mailserver postfix/smtp[17425]: 2BF1C23A7B: to=<john.doe@example.com>, relay=mx.example.com[192.0.2.5]:25, delay=1.2, status=sent (250 2.0.0 OK)

The low-down is as follows:

• Date: Jul 8 10:23:14
• Process ID: postfix /smtp[17425]
• Message 2BF1C23A7B
• Recipient: john.doe@example.com
• Relay: mx.example.com[192.0.2.5]
• Delayed: 0.012 seconds
• Status Sent successfully (250 2.0.0 OK)

These lines will give the precise path that was taken by the email, the duration that it took, and even if there were any delivery errors that appeared.

Red Flags Investigators Look for Understanding SMTP Logs

When people conduct a digital forensic investigation, they are excavating everywhere:

• Not standard IP addresses (those of unknown origin, foreign or anonymous proxies)
• An array of failed hitches in login drives or against SMTP auth
• Actively sending a load of mail through personal accounts
• Delivery failures with messages to the effect that the recipient does not exist (a possible problem with typo squatting)
• Fake main fields in sender email addresses of other domains / IPs
• Indicators of a phishing campaign, e.g., messages were relayed through third-party and unauthorized SMTP servers, or spoofed.

That is the abridged list, and it still covers a lot of ground.

SMTP Log Analyzer Tools

When you are up to the knees in a huge investigation, it is not going to work to scroll through logs after logs. That is why the insiders take out their Sigordon-style toolkit and take out such SMTP log analyzer tools as:

• Microsoft Log Parser
• Splunk
• ELK Stack (Elasticsearch, Logstash, Kibana)
• Forensic email collector (FEC)
• Regular expressions or user-defined Python code

Challenges in SMTP Log Analysis

Nevertheless, working with SMTP logs is not a breeze. You are dealing with:

• Volume: Large companies shoot out millions of lines every day.
• Retention SMTP messages will not be permanently kept in all servers.
• Format Variation: All MTA records events differently, and this may be difficult to compare.
• Transport Layer Security (STARTTLS): This encryption can conceal important metadata.
• Spoofed Headers: In case of spoofed headers, data on the server-side is more stable than that on the client-side.

In short, go to the logs early and compare them with as many sources as possible

Final Thoughts

Understanding SMTP logs? Yep, they are an absolute email forensics gem. Although phishing headers or strange message body changes may give the wrong scent, the plain Logs remain truthful. Bury into them and you will see how a message jumps between the sender and recipient, transforming what seems like a chaotic mess into a bare-bones timeline. Being able to read those Logs will allow you to identify suspicious activity, separate the wheat from the chaff and find the truth hidden in the email breadcrumbs.
When you are up to the knees in cybersecurity or forensics work, knowledge of SMTP logs is not a luxury; it is a necessity.