8 Signs Your Email Has Been Hacked (& How to Fix)

Email compromise often starts with subtle warning signs like messages you didn’t send, login alerts you don’t recognize, or strange reports from coworkers and contacts. Because attackers move quickly once they gain access, spotting these red flags early is critical to protecting your data, reputation, and organization. Whether the breach stems from a phishing email, reused passwords, or an infected device, knowing what to look for can help you respond before the damage spreads. Below are eight clear signs your email may have been hacked, and what to do immediately to secure your account.

1. You Notice Unfamiliar Sent Messages

One of the most frequent warning signs of compromise is unusual activity in your Sent or Outbox folders. Attackers often use a hijacked inbox to send malicious links or fraudulent messages to your contacts. These messages may request money, impersonate a trusted brand, or include attachments designed to install malware.

In many cases, hackers hide their activity by setting up auto-forwarding rules or filters that delete evidence of outgoing messages. This allows them to continue operating in the background without immediate detection.

What to do:

• Check your Sent and Outbox folders for messages you didn’t send.
• Review your email rules and forwarding settings for unknown addresses.
• Reset your password from a trusted device and remove unauthorized connections.

Mimecast’s AI-driven email security platform identifies abnormal sending patterns, helping organizations detect and isolate compromised accounts.

2. You Receive Security Alerts or Login Notifications

Modern email providers notify users about suspicious sign-ins, multiple failed login attempts, or logins from new devices and locations. If you receive one of these alerts without having logged in yourself, it may be an early sign of compromise.

These notifications often include details such as IP addresses, browser types, or device information. Attackers commonly test stolen credentials purchased on the dark web or obtained through phishing.

How to respond:

• Review your login history and device access records.
• Enable multi-factor authentication (MFA) immediately.
• Update your recovery email and security questions.

If you manage corporate accounts, report such activity to your security team. Quick action can help contain potential business email compromise (BEC) before sensitive data is exposed.

3. You Can’t Access Your Account

When you suddenly lose access to your email, it may mean that attackers have already taken control. Once inside, they can change passwords, alter recovery details, or disable security features to maintain access.

Lockouts can happen within minutes of compromise, especially if attackers use automated scripts. They may also modify two-factor authentication (2FA) settings or redirect recovery messages to their own inboxes.

Here are some immediate steps you can take if your account has been compromised:

• Use your provider’s official recovery process to regain access.
• Contact your organization’s IT or security team for additional support.
• Once recovered, change all associated passwords and enable MFA.

Mimecast’s platform helps detect compromised credentials through behavioral analytics and integration with identity and access systems, reducing the risk of long-term unauthorized access.

4. Your Contacts Report Strange Emails from You

If friends, colleagues, or customers report receiving suspicious messages that appear to come from your address, it’s a clear sign of compromise. Attackers often exploit the trust between you and your contacts to spread phishing campaigns.

These messages might contain malicious links, fake invoices, or requests for sensitive data. Beyond personal inconvenience, such incidents can cause lasting damage to an organization’s reputation.

Recommended actions:

• Inform all affected contacts that your account may have been hacked.
• Advise them not to click any links or download attachments.
• Scan your system for malware and review your inbox rules.

5. You Notice Unexpected Password Resets or MFA Prompts

If you receive password reset emails or authentication requests that you didn’t initiate, someone may be attempting to access your account. Attackers often trigger these requests to test whether an account is still active or to exploit partial access.

Repeated MFA prompts can also indicate a “push fatigue” attack, where an attacker repeatedly requests authentication hoping the victim will approve out of frustration or confusion.

To protect your account:

• Decline all unauthorized MFA prompts.
• Change your password using a device you know is secure.
• Switch to app-based MFA rather than SMS-based verification.

It’s important to treat these alerts seriously. They signal that your credentials are either exposed or actively targeted.

6. Your Inbox Settings Have Been Changed

Attackers often manipulate inbox settings to maintain long-term access without raising alarms. Common tactics include creating auto-forwarding rules that send copies of your messages to attacker-controlled accounts, deleting specific types of messages, or rerouting emails that contain certain keywords.

Changes in your inbox behavior, such as missing messages or sudden folder reorganization, may point to unauthorized activity.

Steps to resolve:

• Check all filters, forwarding addresses, and recovery options.
• Disable any unknown or suspicious rules.
• Turn on alerts for setting modifications, if available.

Mimecast’s security solutions integrate with email provider APIs to detect these silent manipulations, providing early visibility into abnormal configurations before data loss occurs.

7. You See Unfamiliar Account Activity or App Connections

Many users link their email accounts to productivity apps, project tools, and social platforms. Attackers can exploit these integrations through OAuth tokens, which grant access without needing your password.

This method allows cybercriminals to persist even after you’ve changed your credentials. You might see unfamiliar apps or permissions granted to third-party services you don’t recognize.

• Review your connected applications through your provider’s security dashboard.
• Revoke permissions for all unknown or unnecessary integrations.
• Regularly audit access lists to maintain oversight.

Attackers are increasingly using OAuth-based exploits because they bypass traditional password reset processes. Even if you secure your account, these hidden tokens can allow ongoing access. Reviewing app permissions regularly is one of the most overlooked but essential steps in maintaining digital hygiene.

8. Your Antivirus or IT Team Flags Unusual Network Behavior

If your antivirus program or IT department reports suspicious network activity, it could indicate a larger compromise. Email attacks often coincide with malware infections or credential-stealing payloads designed to spread laterally across systems.

Unusual traffic patterns, spikes in outbound emails, or repeated communication with unknown domains may signal that an attacker is using your account for broader network exploitation.

Immediate actions:

• Run a full malware scan on all devices connected to your email.
• Isolate affected endpoints to prevent lateral movement.
• Coordinate with IT or cybersecurity teams for log review and incident analysis.

For organizations, correlating email activity with endpoint and SIEM logs provides a clearer picture of compromise. Mimecast’s integrations help security teams analyze behavior in real time and identify linked indicators of compromise, reducing investigation time and response costs.

How to Fix a Compromised Email Account

Once you realize your email has been hacked, the priority now becomes containment and recovery. At this stage, time is of the essence.

Here’s what to do:

1. Change your password immediately using a strong, unique passphrase.
2. Enable multi-factor authentication (preferably through an authenticator app).
3. Review your sent messages, forwarding rules, and connected devices.
4. Notify your IT or security department.
5. Inform your contacts to disregard suspicious messages.

Once your account is secure, assess potential data exposure. Review whether sensitive files, credentials, or attachments were shared. Mimecast’s data protection tools help identify exfiltrated content and assist with post-compromise forensics.

Conclusion

Recognizing the early signs of compromise can make the difference between a quick recovery and a major security incident. From unfamiliar messages to unauthorized settings changes, each signal points to the same underlying question: how do I know if my email has been hacked?

Awareness and rapid response remain the best defenses. If you notice anything unusual, act immediately by securing your credentials, reviewing your settings, and notifying your security team.

Mimecast provides organizations with the tools and intelligence to detect, prevent, and respond to email compromise across all levels. Through AI-powered detection, continuous monitoring, and human-centric risk management, Mimecast helps secure your organization's email and prevent account takeover.

Connect with Mimecast and learn how we can help strengthen your organization's email security.