How healthcare professionals can guard against email bombing

An email bomb attack is almost impossible to prevent because any user with a valid email address can spam any other valid email address. However, there are measures that healthcare professionals can implement to reduce the risk of email bombing and safeguard their email accounts against malicious attacks.

What is email bombing?

Email bombing is a malicious practice in which a large volume of emails are sent to a single email address or a mail server to overwhelm the recipient's inbox or cause disruption to the email service. According to a publication by the Software Engineering Institute, “the messages will be large and constructed from meaningless data in an effort to consume additional system and network resources.” According to the article, “Multiple accounts at the target site may be abused, increasing the denial of service impact.”

Dangers of email bombing

Paubox identifies email as the single largest vector for cyberattacks in the healthcare sector. It is recognized as the industry's top cybersecurity vulnerability and is frequently cited as the weakest security link for healthcare organizations. Since email is such a critical communication channel, attacks that target inboxes can have a disproportionate impact on both individuals and organizational operations. One of the most disruptive forms of attack is email bombing.

According to the study SubStop: An analysis on subscription email bombing attack and machine learning based mitigation, email bombing “cause[s] inconvenience not only to the affected users by overflowing their inboxes but jam the mail servers as well, making it one of the most simplistic yet dreaded forms of a denial of service attack.”

Beyond overwhelming an inbox, email bombing can hide critical account notifications. By flooding a victim’s mailbox with thousands of messages, attackers may hide alerts about suspicious logins, password changes, or financial transactions. This flood of emails acts as a diversion, drawing attention away from malicious activity occurring in the background.

Attackers have been known to purchase pricey items using online shopping accounts they have gained access to, conduct fraudulent transactions on victims' bank accounts, and threaten domain owners with losing their email addresses to force them to give them up.

The study further notes that there is currently no single mechanism or technique capable of fully detecting or mitigating subscription bombing attacks. As a result, individuals and organizations often struggle to manage the overwhelming volume of unwanted emails generated during such attacks, which can significantly disrupt communication, productivity, and system availability.

See also: What is ransomware and how to protect against it

How to guard against email bombing

To address this challenge, the researchers in the study SubStop: An analysis on subscription email bombing attack and machine learning based mitigation propose a layered detection framework called “SubStop,” designed to identify and mitigate email bombing attacks more effectively.

Monitoring sudden spikes in email volume

The study suggests using a time-based monitoring system that compares the current volume of incoming emails with a user’s historical average. If the number of emails received within a short time window, such as 10 minutes, exceeds a predefined threshold, the system flags the activity as a potential bombing attack.

Identifying suspicious sender domains

After detecting unusual traffic, the next step is analyzing where the emails are coming from. The researchers propose an “address locator filter,” which examines the domain portion of the sender’s email address. The system separates emails from new or unknown domains from those sent by trusted or previously known domains, helping security tools isolate potentially malicious subscription emails.

Applying machine learning for classification

The SubStop framework uses a weighted Support Vector Machine (SVM) classifier to analyze patterns in incoming messages and determine whether they are part of a bombing attack. This machine-learning approach prioritizes identifying bomb emails while minimizing the risk of incorrectly flagging legitimate messages.

Training defenses using real user email data

The study also notes that effective detection depends on understanding a user’s normal email behavior. Since legitimate subscriptions vary between individuals, the system improves its accuracy by training models using data from a user’s inbox alongside publicly available datasets. This personalization helps the system distinguish between genuine newsletters and suspicious subscription bursts.

Reducing manual intervention for victims

Without automated detection, victims of subscription bombing must manually unsubscribe from hundreds of mailing lists. The layered filtering architecture proposed in the study automates the identification and categorization of bomb emails, allowing systems to process large volumes of messages quickly and notify users of legitimate communications that may require attention.

How Paubox’s inbound email security can defend against email bombing

Paubox’s inbound email security provides several protective layers that can help organizations defend against email bombing attempts. These include:

AI-driven anomaly detection

Paubox uses generative AI to analyze the behavior, tone, and intent of incoming emails and compare them against normal communication patterns. If the system detects unusual patterns, such as sudden bursts of emails from unfamiliar sources, it can flag those messages as suspicious and prevent them from reaching the inbox.

Sender validation and reputation checks

Each inbound email is evaluated to confirm the legitimacy of the sending domain and mail server. This includes validating domain authentication records and reviewing the sender’s reputation. Emails that fail these checks are rejected or blocked before delivery.

By filtering out suspicious or unknown senders early in the process, organizations can reduce the number of malicious emails that contribute to inbox flooding.

Advanced spam filtering and AI classification

Paubox combines AI analysis with traditional filtering to classify incoming messages based on metadata, content patterns, and sender behavior. Suspicious emails are automatically flagged and filtered before reaching the user.

This layered filtering approach helps identify large volumes of unwanted messages, such as those generated during subscription-based email bombing attacks.

Quarantine management and threat isolation

Emails identified as spam, phishing, or otherwise suspicious are placed in a quarantine environment rather than delivered to the inbox. Administrators can review quarantined messages through the security dashboard and decide whether they should be released or deleted. Quarantining suspicious messages prevents inbox overload while still allowing IT teams to investigate potential attacks.

Customizable blocking rules and geofencing

Organizations can create custom rules to block emails from specific domains, IP addresses, or regions associated with suspicious activity. Paubox also offers geofencing capabilities, enabling administrators to block inbound emails originating from certain countries known to generate large volumes of spam or phishing attempts.

Go deeper: HIPAA Compliant Email: The Definitive Guide

What to do during an email bombing attack

According to the CERT Coordination Center guidance from the Software Engineering Institute article, organizations should:

Identify the source of the attack

“Identify the source of the email bomb/spam and configure your router (or have your Network

Service Provider configure the router) to prevent incoming packets from that address.”

Block malicious senders at the network level

After identifying the source, organizations should implement network or mail-server filtering rules. These rules can block packets or messages from the attacking address before they reach the inbox.

Notify affected organizations and service providers

The CERT guidelines advise contacting the organizations connected to the questionable domains or servers because attackers frequently try to conceal their identities. Informing these sites about the activity allows them to investigate potential abuse of their systems and may help stop the attack at its origin.

Document the incident and report it

The CERT Coordination Center suggests keeping records of the attack and reporting the activity to relevant incident response teams.

Update email software and strengthen monitoring

Organizations should ensure that their email delivery software is fully updated and that logging capabilities are enabled.

Go deeper: How to survive an email bomb attack

FAQS

Why do attackers use email bombing?

In many cases, email bombing is used as a distraction tactic to hide security alerts such as password change notifications or transaction confirmations. It can also be used as a denial-of-service (DoS) attack to disrupt communication systems or overwhelm email servers.

What are the warning signs of an email bombing attack?

Common warning signs include:

• Receiving hundreds or thousands of emails within minutes
• Multiple subscription confirmation emails from unfamiliar websites
• Email services becoming slow or temporarily unavailable
• Difficulty locating legitimate messages among the incoming emails