Detecting and Blocking Compromised Accounts

One compromised email account can do more than just send spam—it can tarnish your sender reputation, damage your IP trustworthiness, and trigger blacklisting that affects thousands of users. If you’re managing email infrastructure for a hosting platform, ESP, or SaaS product, early detection of compromised accounts is critical to maintaining email deliverability.

In this post, we’ll break down how to identify compromised accounts and block them before they hurt your network.

What Is a Compromised Email Account?

A compromised account is any user account that has been hijacked—usually via phishing, credential stuffing, or CMS vulnerabilities—and is being used to send malicious email.

Once compromised, accounts are typically used to send:

• Spam campaigns (e.g., pills, crypto, adult content)
  
• Phishing emails designed to steal further credentials
  
• Malware or links to infected websites
  
• Bulk mail to purchased lists to test deliverability or evade filters
  

How to Detect Compromised Email Accounts

1. Monitor for Volume Spikes

A sudden increase in outbound email volume—especially from dormant or low-usage accounts—is a red flag. Set per-user and per-domain baselines, and trigger alerts when activity deviates significantly.

2. Look for SMTP Auth Failures

An uptick in failed login attempts may signal brute-force attacks or bots trying to access the account. Block repeated failures at the IP level and implement rate-limiting.

3. Analyze Email Content

Scan outgoing messages for:

• Phishing links
  
• Blacklisted domains
  
• Common spam keywords
  
• Unusual headers or forged sender addresses
  

Related: Using AI & Heuristics for Spam Detection and Prevention

4. Identify New or Suspicious Sending Patterns

Has a user suddenly started sending from a new location or IP block? Is the account suddenly sending to a large list of unrelated recipients? These behavioral anomalies are often signs of compromise.

5. Use Feedback Loops & Complaint Data

Sign up for ISP feedback loops (FBLs) to receive complaint reports. High complaint rates tied to specific accounts can help you take quick action.

Tip: Use MailChannels ResponseAnalytics to detect patterns tied to IPs, users, and domains in real time.

How to Block and Contain the Threat

1. Disable the Account Immediately

Once flagged, disable sending privileges and log out all active sessions. Notify the account owner of the suspicious activity.

2. Require Password Reset + 2FA

Force a password change and, ideally, enable two-factor authentication to prevent future compromise.

3. Quarantine Suspicious Emails

Move suspicious messages into a holding queue or discard them before delivery. This protects your IP reputation while giving you time to investigate.

4. Block Abuse at the SMTP Layer

Use smart SMTP relays like MailChannels to filter malicious content at the transport level, before it damages inbox trust.

Preventing Future Compromises

• Enforce strong passwords and rate limits
  
• Disable PHP mail() for shared hosting
  
• Restrict API keys to specific IPs or functions
  
• Audit plugin usage on CMS sites (e.g., WordPress)
  
• Educate users about phishing and email security
  

Explore: Best Practices to Prevent Outbound Spam

How MailChannels Helps

MailChannels detects abnormal sending behavior using real-time analytics, machine learning, and reputation tracking. Compromised accounts are blocked before they can impact your deliverability.

Stop threats before they spread.
Get protected with MailChannels Outbound Filtering

Key Takeaways

• Compromised accounts are a leading cause of spam and blacklisting.
  
• Use behavioral analysis, content scanning, and complaint data to detect issues early.
  
• Isolate and block suspicious accounts immediately.
  
• Smart SMTP relays like MailChannels help automate protection and minimize risk.
  

Want to reduce abuse and protect your IPs?
Let MailChannels handle outbound spam detection