Outbound phishing is a nightmare scenario.
This is when a hacker hijacks one of your own legitimate user accounts—like a Microsoft 365 or Google Workspace inbox—and uses it as their personal, malicious megaphone.
They start blasting phishing emails from your trusted domain to everyone you do business with. Your customers, your partners, and your own employees all become targets.
Why this is a five-alarm fire
An outbound phishing attack is not a minor problem. It is a critical, hair-on-fire incident for three reasons.
It torches your reputation: First, it destroys the trust you've built. When your customers and partners get scammed by an email from you, your brand is damaged in a way that is incredibly hard to repair.
It gets you blocklisted: Email service providers and security filters will see your domain sending malicious content. They will quickly add your domain to global blocklists, meaning even your legitimate emails—invoices, quotes, support replies—will stop getting delivered.
It's a symptom of a deeper sickness: This is the most important part. Outbound phishing is a flashing red sign that you are already breached. The attacker isn't at the gates. They are in your house, using your tools, and they have been for some time.
How this even happens
Outbound phishing is the result of a different, earlier attack. The chain of events almost always looks like this:
The initial compromise: A user in your organization falls for a different phishing email, reuses a password, or gets hit with an MFA-bypass attack.
The account takeover: The attacker successfully steals their credentials and takes control of their account.
Weaponization: The attacker now "owns" a trusted, legitimate account. They often create "shadow workflows" by setting up inbox rules to auto-delete replies, so the real user stays in the dark. Then, they begin their attack, leveraging the full trust of your brand.
The attacker's playbook
Once they have control, hackers get creative. They don't just send obvious, generic spam.
They will reply to existing, legitimate email threads to add a sense of urgency and authenticity. They'll drop a "Here's the updated invoice" or "Please review this new contract" link into a conversation you were just having.
This is a classic, devastating tactic used in Business Email Compromise (BEC) attacks, which the FBI identifies as a multi-billion dollar problem. This is how they trick your partners into wiring money to the wrong bank account or your customers into giving up their own credentials.
In conclusion
Outbound phishing is one of the clearest and most dangerous signals of a deep, active compromise.
It proves that your initial prevention (like spam filters) has failed, and it highlights a critical need for modern security that detects a breach in progress. This is why Identity Threat Detection and Response (ITDR) is so critical. You must have a way to spot the suspicious behavior of a compromised account before it burns your reputation to the ground.
FAQs
Inbound is what you receive. It's the classic phishing email from a stranger that lands in your inbox. Outbound is what you send. It's a phishing email sent from your company's legitimate, hijacked account to your contacts.
Look for suspicious activity:
A "Sent Items" folder full of emails you never wrote.
Replies from people you don't know, or who are confused about an email you "sent."
New, strange inbox rules (e.g., "auto-delete all replies") that you didn't create.
Sudden "email not delivered" bounce-back messages in large volumes.
Contain it: Immediately change the password for the compromised account.
Kick them out: Force a sign-out of all active sessions for that user.
Investigate: Check for any new inbox rules, forwarded emails, or other malicious activity. This is where an IT professional or security partner is critical.
Communicate: You must inform your customers and partners that you were compromised and warn them to be suspicious of recent emails.
MFA is your single best preventative tool against the initial account takeover. However, it is not a silver bullet. Attackers can (and do) bypass it with techniques like session token hijacking or MFA fatigue bombing. You still need detection for what happens after a bypass.
Global spam-fighting organizations (like Spamhaus) and email providers (like Google/Microsoft) see a high volume of malicious mail coming from your domain. To protect their other users, they add your domain to a blacklist, which tells all other mail servers not to trust or deliver email from you.
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