BY - Dashrath Jamadar

Email Gateway for Security & Monitoring

1. Definition of an Email Gateway

An Email Gateway is a security solution that sits between an organization’s email infrastructure and the internet, filtering
incoming and outgoing emails to detect and block threats, enforce policies, prevent data leaks, and ensure compliance.

It protects against:
Phishing (fake emails trying to steal credentials)
Malware (emails with infected attachments or links)
Spam & Graymail (unwanted bulk emails)
Data Loss (DLP) (prevents sending sensitive information externally)
Spoofing & Impersonation Attacks (fake emails pretending to be from trusted people)

2. How an Email Gateway Works

Step-by-Step Process:

1. Inbound Email Filtering (Emails coming into the company)

o Checks email headers for spoofing and impersonation.

o Scans attachments and links for malware and phishing.

o Uses threat intelligence to block known malicious senders.

o Marks suspicious emails for user awareness (e.g., "[External Sender]").

2. Outbound Email Filtering (Emails sent out by employees)

o Prevents sensitive data leaks (e.g., sending customer data outside).

o Blocks sending of malware-infected attachments.

o Enforces company policies (e.g., no personal email use for work files).

3. Quarantine & Policy Enforcement

BY - Dashrath Jamadar

o Suspicious emails are held for review (not delivered immediately).

o Admins & SOC teams get alerts for high-risk emails.

o Users can release false positives from quarantine.

4. Logging & SIEM Integration

o All email events (blocked, delivered, flagged) are logged.

o Email Gateway sends logs to SIEM for correlation with other security events.

3. Log Formats Supported by Email Gateways

Email gateways generate logs in multiple formats:

• Syslog (Standard log format for SIEM ingestion)

• Common Event Format (CEF)

• Log Event Extended Format (LEEF)

• JSON (For API-based integrations)

• Plain Text (CSV, TXT)

4. Logs Generated and Sent to SIEM

Common Email Gateway Logs Sent to SIEM:

1. Email Delivered Logs – Details of successfully delivered emails.

2. Blocked Email Logs – Information about phishing, malware, or spam emails blocked.

3. Quarantine Logs – Emails held for review before delivery.

4. Attachment Scan Logs – Results of file scans (e.g., clean, malicious).

5. URL Click Logs – Tracks if a user clicked a link in an email.

6. DLP Violation Logs – When an employee attempts to send sensitive data outside.

5. Advantages of Using an Email Gateway

Protects Against Phishing & Spoofing – Detects fake emails trying to steal credentials.
Stops Malware & Ransomware – Blocks infected attachments and malicious links.
Reduces Spam – Filters out unwanted bulk emails.
Prevents Data Leakage – Stops sending of sensitive files outside the company.
Ensures Compliance – Helps meet PCI DSS, HIPAA, GDPR security policies.
Enhances Visibility – Provides logs for SOC teams to investigate threats.
Integrates with SIEM – Correlates email events with other security incidents.
BY - Dashrath Jamadar

6. Disadvantages of Email Gateways

False Positives – Can block legitimate emails mistakenly.

Latency Issues – Email delivery may be slightly delayed due to scanning.
Bypass Risks – Attackers may use encrypted attachments to avoid detection.
High Maintenance – Requires constant updates to detect new threats.
SOC Team Overload – Too many quarantine alerts if not fine-tuned properly.

7. Email Log Flow to SIEM

1. Incoming Email Received → Email gateway scans and logs it.

2. Threat Detected?

o Yes → Blocked & Logged (sent to SIEM)

o No → Delivered normally

3. Outbound Email Sent → Scanned for DLP violations before sending.

4. SOC Team Investigates Suspicious Emails → Uses SIEM logs for deep analysis.

Example SIEM Use Case:

• Scenario: An employee receives a phishing email with a fake login page link.

• Email Gateway Action:

o Blocks the email and logs it.

o Sends an alert to SIEM for correlation with other security threats.

o SOC team investigates if similar emails were received by other employees.

8. Top Email Security Gateway Vendors in 2025

1. Proofpoint Email Security

2. Mimecast Secure Email Gateway

3. Microsoft Defender for Office 365

4. Cisco Secure Email (IronPort)

5. Barracuda Email Security Gateway

6. Fortinet FortiMail

7. Trend Micro Email Security

8. Symantec Email Security (Broadcom)

9. Zscaler Email Security

10. Google Workspace Enterprise Email Protection

BY - Dashrath Jamadar

9. 12 Use Cases for an Email Gateway

1. Phishing Detection & Blocking

Detect and block emails with suspicious links, spoofed domains, or credential harvesting content before reaching users.

2. Business Email Compromise (BEC) Prevention

Flag and quarantine emails where attackers impersonate executives or vendors requesting fund transfers or sensitive data.

3. Malware Attachment Filtering

Block emails carrying malware payloads (e.g., .exe, macro-enabled Word/Excel files) using sandboxing and signature-based
scans.

4. URL Rewriting & Time-of-Click Protection

Rewrite embedded links in emails and analyze them at click-time to prevent delayed phishing/malware attacks.

5. Spam & Bulk Mail Filtering

Automatically detect and filter unsolicited or mass marketing emails to reduce noise and protect end-users.

6. Zero-Day Exploit Mitigation

Use machine learning and sandboxing to identify and block unknown threats embedded in emails before signatures are
available.

7. DMARC, DKIM, SPF Validation

Reject spoofed emails by enforcing authentication policies using DMARC, DKIM, and SPF records.

8. Impersonation Attack Detection

Identify and alert on emails mimicking trusted senders (e.g., lookalike domains or display name spoofing).

10. Sensitive Data Leakage Prevention

Prevent accidental or intentional sharing of PII, credit card info, or proprietary data via outbound emails using DLP policies.

11. Threat Intelligence Integration

Correlate email IOCs (URLs, hashes, IPs, domains) with threat feeds and SIEM to identify targeted campaigns or actor TTPs.

12. File Type and Extension Restrictions

Block non-approved file types (e.g., .js, .vbs) even if renamed or zipped.

✅ Backend Process of an Email Gateway (Incoming Email Focus)

Applies to tools like: Microsoft Defender for Office 365, Proofpoint, Mimecast, Cisco ESA, etc.
1. Connection & SMTP Session Handling
When an external mail server sends an email to your organization:
• The email gateway accepts the SMTP connection on port 25 (SMTP) or 587 (SMTP Auth).
BY - Dashrath Jamadar

• It verifies the sending server's IP and applies rate limiting or reputation checks.
Checks performed:
• Sender IP reputation (via RBLs like Spamhaus, Talos, etc.)
• HELO/EHLO domain validity
• SMTP command sequence compliance
• TLS encryption enforcement

If the IP or connection is suspicious, the gateway may drop it immediately before content is processed.

2. Envelope and Header Filtering (Before Body is Analyzed)

• The email gateway extracts envelope details (MAIL FROM, RCPT TO) and headers like:
o From, Reply-To, Return-Path
o Received, Message-ID
• SPF, DKIM, and DMARC checks are applied here.
Checks performed:
• SPF: Is the sending IP allowed to send on behalf of the domain?
• DKIM: Does the digital signature match?
• DMARC: What should be done if SPF/DKIM fails?
Helps identify spoofed or forged emails before scanning the message body.
3. Content Filtering & Malware/Phishing Inspection
Now the gateway inspects the email body, attachments, and embedded URLs.
a. Antivirus/Antimalware Scanning
• Uses signature-based engines and hash checks.
• Scans attachments, embedded files, and links for known threats.
b. Advanced Threat Protection (ATP/Sandboxing)
• Suspicious files/links are detonated in a sandbox environment.
• Emulates file behavior:
o Does it connect to C2?
o Try to drop a payload?
o Show fake login pages?
c. URL Rewriting/Scanning
• All URLs are rewritten to pass through a secure filter (e.g., https://security.company.com?url=...)
• URLs are scanned in real time when the user clicks.
BY - Dashrath Jamadar

d. Natural Language Processing (NLP)

• Email content is analyzed for phishing tone, urgency, or social engineering patterns.

Multiple detection layers (signature, sandbox, heuristics) are used to catch both known and unknown
threats.
4. Policy Enforcement & Routing Decision
After scanning, the gateway applies organizational email policies such as:
• Block/quarantine/delivery based on:
o File types (e.g., .exe, .zip, .js)
o Threat level (High, Medium, Low)
o BEC or impersonation detection
• Tagging:
o Add custom headers like X-Proofpoint-Spam: high
o Modify subject line (e.g., [EXTERNAL], [PHISHING])

Ensures that clean emails are delivered, and suspicious ones are quarantined or modified for awareness.
5. Delivery to Mailbox or Quarantine
Depending on the verdict:
• Safe emails: Delivered to Exchange Online, Gmail, or internal mail servers.
• Suspicious or malicious emails:
o Sent to Quarantine portal for user/admin review.
o Or dropped silently (based on policy).
User may receive:
• Quarantine digest
• Alert (optional)
• Link to release/review email
6. Logging, Alerting, and SIEM Integration
All actions are logged for SOC visibility.
Logs contain:
• Envelope + header + verdict
• Threat name/type (if detected)
• Sandbox results (hashes, behavior)
• Final action taken
• User interaction data (if clicked)
These logs are:
BY - Dashrath Jamadar

• Pushed to SIEM (e.g., Microsoft 365 logs to Sentinel/Exabeam/QRadar)

• Used for correlation rules and alert generation
Allows SOC teams to investigate phishing, malware, BEC, or insider threat incidents.

🔚 Final Flow Summary:

csharp
CopyEdit

[Incoming Email] ➜

[SMTP Connection Checks] ➜

[Header & Auth Checks (SPF/DKIM/DMARC)] ➜

[Content + Attachment Scanning] ➜

[URL Rewriting & Sandboxing] ➜

[Policy Application] ➜

[Delivery or Quarantine] ➜
[Logging to SIEM]

Scenario: Suspicious File Upload Detected via Email Gateway Logs

Question:
A SIEM alert was triggered when the email gateway detected a user attempting to send multiple sensitive documents to a
personal or unauthorized external email address. Walk me through how you'd validate, investigate, and respond to this using
only email gateway logs and related SOC tools.

Step 1: Validate the SIEM Alert from Email Gateway Logs

Initial Alert Validation:

• Navigate to the SIEM dashboard and locate the triggered rule.

• Review the correlated log from the email gateway source (e.g., Proofpoint, Mimecast, Microsoft Defender for Office
365, etc.).

Key log details to analyze:

• Sender's email address

• Recipient's domain/email (e.g., @gmail.com, @yahoo.com – indicates personal use)

• Subject line (does it indicate confidentiality or sensitive topics?)

• Attachment details (name, size, file type: .xls, .pdf, .zip)

BY - Dashrath Jamadar

• Timestamp of the activity

Why this is important:

Determine if this action violated data transfer policies, such as sending sensitive files (PII, financial data) to non-corporate
addresses.

Step 2: Deep Dive into Email Gateway Log Correlation

Email Gateway Tools (e.g., Proofpoint, Microsoft 365 Defender):

• Search for all emails sent by the user in a specific timeframe (e.g., past 24–48 hours).

• Filter logs for messages with:

o Attachments

o External recipients

o Subject/Body with keywords like “confidential,” “report,” “salary,” etc.

Actions to take:

• Check if multiple recipients were involved (could point to data spamming).

• Identify if auto-forwarding rules are set on the user’s mailbox.

Why this is important:

Helps determine whether this was a one-time error, recurring pattern, or part of data exfiltration.

Step 3: Analyze Attachment Metadata in Logs

Examine attachment details:

• File names and sizes

• MIME types (e.g., application/pdf, application/vnd.ms-excel)

• Frequency: Are the same files being sent to multiple destinations?

Why this is important:

Large or compressed file attachments sent to unauthorized domains might indicate intentional data leakage.

Step 4: Cross-Check Email Header Details

From the email log:

• Analyze the Return-Path, Reply-To, and Received headers.

• Look for signs of:

o Spoofing

o Unauthorized relay

o Use of proxy or VPN IPs in message transmission

BY - Dashrath Jamadar

Step 5: Correlate with User Behavior in SIEM/UEBA (If Available)

If UEBA integration exists:

• Check for:

o Sudden change in user behavior (e.g., emailing after-hours, increased data transfers)

o New external domains not contacted before

• Is the user from a department that should handle sensitive data?

Why this is important:

This gives context on whether the user typically sends such emails or if this is abnormal behavior.

Step 6: Response and Containment

Immediate Actions:

• Quarantine the email if still in queue (email gateway action).

• Block the recipient domain temporarily via the email gateway.

• Notify IT or AD team to:

o Lock the user account (if malicious intent is suspected)

o Remove any auto-forwarding rules or delegations

• Alert DLP or compliance if sensitive data was confirmed.

Why this is important:

Limits further spread or leaks and preserves evidence.

Step 7: Escalation and Documentation

Escalate to:

• HR (if insider threat is suspected)

• Legal/Compliance (especially for PII, GDPR, HIPAA violations)
• Line manager or department head

Document:

• SIEM alert details

• Email logs summary
• Attachment metadata
• Timeline of events
• Actions taken (quarantine, block, lockout)
• Recommendations for policy change (if needed)

Step 8: Root Cause Analysis and Prevention

BY - Dashrath Jamadar

Remediation steps:

• Update email gateway rules:

o Block sending to public domains with attachments by default.

o Add keyword/content filters for sensitive data.

• Enforce DLP at the email level (if not already in place).

• Conduct user awareness training for secure data handling.

Scenario: Suspicious Incoming Email Detected via Email Gateway Logs

Question:
A SIEM correlation rule triggers when the email gateway flags an incoming email containing a potentially malicious
attachment sent to an internal user. Walk me through how you would validate, investigate, and respond using only email
gateway logs and related SOC tools.

Step 1: Validate the SIEM Alert from Email Gateway Logs

Review the SIEM Alert:

• Access the SIEM dashboard and locate the alert triggered by the email gateway log source (e.g., Proofpoint,
Mimecast, Microsoft 365 Defender).

• Confirm the alert is based on a malicious indicator such as:

o Malicious attachment (e.g., .exe, .js, .docm, .zip)

o Suspicious sender reputation or domain
o Known IOC (hash, sender IP, domain)

Key fields to review in the log:

• Sender’s email address and IP

• Recipient (internal user)
• Subject line
• Attachment name and type
• Timestamp
• Verdict from the email security engine (e.g., Malware Detected, Spam, Phishing)

Purpose: Ensure this is not a false positive and determine the risk level of the incoming email.

Step 2: Investigate the Sender Reputation and Source

Sender Domain & IP Analysis:

• Check if the sender domain is newly registered, spoofed, or similar to a legitimate domain (e.g., micr0soft.com).

• Analyze the source IP address:

o Use threat intel platforms (VirusTotal, AbuseIPDB, Talos) to check if it’s a known malicious sender.
BY - Dashrath Jamadar

• Review SPF, DKIM, DMARC results from the email log headers:

o Failed checks could indicate spoofing or unauthenticated sending.

Purpose: Assess trustworthiness of the sender and confirm if the email bypassed security policies.

Step 3: Analyze the Email Content and Attachments

Attachment Metadata Review:

• Examine:

o File name, size, hash

o File type (e.g., macro-enabled Word files, scripts, executables)
o Is the extension hidden (e.g., .pdf.exe)?

Advanced: If sandboxing is enabled in the gateway (e.g., Proofpoint TAP, Defender ATP Safe Attachments):

• Retrieve sandbox analysis report:

o Execution behavior
o Callback URLs
o File actions (e.g., attempts to drop another file or connect to C2 server)

Purpose: Identify if the attachment is designed for malware delivery or phishing credential harvesting.

Step 4: Identify Affected Users and Email Spread

Check Recipient Logs:

• Was the email sent to multiple users or just one?

• Use email gateway or SIEM to filter messages by:

o Same sender address

o Same subject
o Same attachment hash

Purpose: Determine if this is a targeted spear-phishing or a widespread campaign.

Step 5: Response and Containment Actions

Immediate Containment:

• Quarantine the email if not already blocked.

• Delete or retract the email from mailboxes (e.g., Microsoft Purview Search & Purge, Proofpoint quarantine actions).

• Block:

o Sender domain/IP
o Attachment hash
o Any URLs found in the email
BY - Dashrath Jamadar

If users clicked a link or opened the attachment:

• Notify the EDR team to scan the endpoint.

• Look for lateral movement, suspicious processes, or malware payloads.

Purpose: Prevent user compromise and halt any ongoing attacks.

Step 6: Escalation and Documentation

Escalate to:

• Threat Intel team (for IOC enrichment and blocklisting)

• IT/Desktop team (if user opened file)
• Management (if it's a targeted phishing campaign)

Document:

• SIEM alert
• Email metadata and attachment details
• Sender IP/domain intelligence
• Actions taken (quarantine, block, user notification)
• Timeline of the event

Purpose: Ensure full visibility and audit trail for post-incident review and compliance.

Step 7: Post-Incident Actions and Prevention

Strengthen Controls:

• Adjust email gateway policies to:

o Quarantine or block suspicious file types by default

o Increase sensitivity of spam/phishing rules

• Enable URL rewriting and attachment sandboxing (if not enabled)

• Set strict SPF, DKIM, DMARC enforcement

Awareness:

• Notify impacted users

• Send security bulletin about recent phishing techniques
• Recommend not opening attachments from unknown senders