Email Phishing Investigation 101: Best Practices

The Challenge

Email is still the easiest path into a company, leading to billions in annual losses across organizations. Secure email gateways and user reports generate large queues of suspected phishing. The signal is noisy and full of duplicates. Analysts must separate true phishing from spam and unwanted mail, determine who interacted with the message and to what extent (clicks, attachment download, etc), and confirm whether controls were bypassed. They need to do this quickly.

This work competes for attention with incidents that show clearer severity. Phishing alerts often look routine until they are not, so they slip behind while shared mailboxes grow. Backlogs delay containment and raise exposure. Teams need a reliable way to rank what to investigate, scope the blast radius across recipients and mail flow, and act before there is business impact.

The process of investigating a reported phishing email follows a methodical assessment of the message headers, content, and attachments.

The email header contains the technical metadata and routing information for a message, serving as a critical forensic trail. A security analyst will often assess:

• Authentication Results: The results of email authentication protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance). A "Fail" result for any of these checks is a strong indicator of a spoofed or malicious sender.
• Received: This field is a log of every server the email passed through from its origin to the recipient's inbox. Reading these entries from bottom to top allows the analyst to trace the email's true path. Discrepancies between the sender's claimed domain and the originating IP address in this log are a clear red flag.
• Return-Path: This specifies where undeliverable messages should be sent. In a phishing attempt, this address may be different from the visible "From" address and lead to a suspicious, unrelated domain.
• Message-ID: A unique identifier for the email. Analyzing this across multiple reported messages can help identify a mass phishing campaign.
• X-Headers: Many email services and security tools add custom headers (X-Originating-IP, X-Spam-Status) that provide additional information on the sender's origin and the email's spam score.

Email attachments, when included, are also an investigation target. Attachments are a primary delivery vector for malware. Investigating attached files would include:

• Verifying File Types and Names: Deceiving file extensions are a common factor in attached malware. For instance, a file named invoice.pdf that is actually invoice.pdf.exe is a common and obvious red flag. Suspicious file types include executables (.exe), compressed files (.zip), and documents with macros (.docm).
• File Behavioral Analysis (Sandboxing): To safely inspect an attachment, analysts would often open the file in an isolated and instrumented environment called a sandbox. This environment executes the file and monitors its behavior, looking for malicious actions such as connecting to command-and-control servers, attempting to install malware, or encrypting files. This analysis can provide a definitive verdict on whether the attachment is malicious without posing a risk to the organization's network.

When a phishing email is delivered to the user's inbox it’s critical to verify if they had any interaction with the message. This step moves from analyzing the email itself to examining the user's environment and activity:

• Email Communication Logs: Checking the user's sent mailbox to see if they replied to the phishing email. A reply could indicate that the user has provided sensitive information or engaged with the attacker.
• Network and Proxy Logs: The analyst reviews network logs to identify any connections from the user's device to the malicious URLs found in the email. A successful connection is a strong indicator that the user clicked a link.
• Endpoint Detection and Response (EDR) Telemetry: EDR systems provide detailed logs of activity on a user's machine. The data can be used to determine if an attachment from the suspicious email was opened, executed, or if any subsequent malicious processes were initiated.
• User Account Activity: Examining the user's account for any signs of compromise, such as new login sessions from unusual geographic locations, new mail forwarding rules created in their mailbox, or unauthorized access to corporate resources.

By meticulously performing these steps security teams determine the malicious nature of a reported email, assess the level of risk and take appropriate remediation actions. The steps to investigate and respond to phishing are well known and are usually well documented by security teams, but due to the volume of cases, number of steps and complexity, these tasks demand a considerable amount of resources that most organizations simply do not have available.

How Prophet Security helps

Prophet AI removes manual work from email phishing response. It triages every alert on arrival, enriches with threat intelligence, analyzes content, headers, links, and attachments, checks delivery and quarantine status, validates user and account activity, and maps blast radius across the tenant.

The platform correlates evidence from your mail gateway, identity logs, audit trails, and endpoint context to produce an evidence backed determination with recommended or automated actions in minutes. Your team clears backlogs and focuses on real risk reduction instead of repetitive tasks. Request a demo of Prophet AI to see it in action. 

Frequently Asked Questions (FAQ)

What is an email phishing investigation?

An email phishing investigation refers to the process of validating whether a reported email is malicious and scoping its impact. It starts with header, content, and attachment analysis, then traces delivery, user interaction, and any execution on endpoints. The goal is to make a determination and drive containment quickly.

How do analysts use email headers in a phishing investigation?

Analysts use email headers in a phishing investigation to reconstruct the message route and trust signals. They review Authentication-Results for SPF, DKIM, and DMARC, the Received chain, Return-Path, Message-ID, and any X-Headers. Inconsistencies between the claimed domain and the originating IP or failing authentication are red flags.

What do SPF, DKIM, and DMARC results mean in email phishing investigations?

SPF, DKIM, and DMARC results in an email phishing investigation indicate whether the sender was authorized and the message was tamper free. A fail or misalignment across any protocol raises the likelihood of spoofing, while full alignment improves confidence but does not guarantee safety. Analysts weigh these results with other forensic evidence.

How should attachments be analyzed safely during a phishing investigation?

Attachments in a phishing investigation should be inspected by verifying file types and names, then executing suspicious files only in a sandbox. Analysts look for behaviors such as external callbacks, process spawning, or encryption attempts. They combine static file metadata with sandbox results to reach a verdict.

How do you confirm whether a user interacted with a phishing email?

Confirming whether a user interacted with a phishing email involves correlating mail logs, proxy or network logs, and EDR telemetry. Investigators check for replies, link clicks, attachment opens, new processes, and mailbox rule changes or unusual sign ins. Verified interaction raises priority and shapes containment.

How do you scope the blast radius of a phishing campaign across a tenant?

Scoping the blast radius of a phishing campaign means mapping delivery, forwarding, and replies across recipients and mail flow. Analysts pivot on Message-ID, sender, subjects, and URLs to find related messages, then evaluate which users clicked or executed content. The output is a list of affected accounts, devices, and systems for containment.

What metrics show the impact of improving email phishing investigations?

Metrics that show the impact of improving email phishing investigations include dwell time for phishing alerts, mean time to investigate, mean time to resolve, backlog size of reported emails, false positive rate, and investigation throughput. Tracking these over time shows whether triage and containment are accelerating and backlogs are shrinking. Many teams also track the percentage of alerts auto triaged with human verified accuracy.

How can an AI SOC platform help manage phishing alert backlogs without losing control?

An AI SOC platform can help manage phishing alert backlogs by automating evidence collection, header and content analysis, enrichment, and cross tool correlation to produce an evidence backed determination. Control is preserved with policy gates for proposed actions, human approvals for risky steps, and full audit trails of queries, evidence, and timestamps. This reduces manual steps while keeping analysts in charge.