Outbound phishing 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.
Key Takeaways
- Trusted sender abuse: Outbound phishing weaponizes the victim's own domain recipients trust messages from known contacts, dramatically increasing click-through rates on malicious links
- Domain blocklisting risk: When email providers detect spam originating from your domain, they add it to global blacklists, blocking all legitimate business email until resolved
- Compromise signal: Outbound phishing indicates attackers have already established persistent access inbox rules and shadow workflows are typically set up before sending begins
- Attacker stealth: Threat actors reply to existing email threads with messages like 'Here's the updated invoice,' exploiting established trust rather than sending obvious cold spam
- Immediate response: Change compromised passwords, force sign-out of all sessions, audit mailbox rules and forwarding settings, and notify affected customers and partners immediately
Outbound phishing vs. inbound phishing: why the distinction matters
Most security awareness training focuses on inbound threats—spotting suspicious emails as they hit the inbox. Outbound phishing flips that script: the threat originates from inside a compromised mailbox. With inbound phishing, your user receives the email; with outbound abuse, your organization’s domain is used to send the attack, making you both the victim and an unwilling participant. That distinction changes how you respond, what you may be obligated to report, and how you communicate with customers and partners.
For MSPs managing client Microsoft 365 environments, the ability to spot suspicious identity and mailbox activity—like new or modified forwarding rules, unusual login locations or VPN usage, and other signs of mailbox tampering—is a concrete service differentiator. Huntress Managed ITDR monitors for exactly this kind of attacker tradecraft, catching compromised accounts before they can be widely abused or cause reputational damage and downstream harm to clients’ customers.
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.
Recovering from an outbound phishing incident
Recovery steps in detail beyond the immediate triage:
- Change the compromised account's password and force session revocation.
- Audit all mailbox rules, forwarding settings, and OAuth app permissions — attackers commonly establish persistence through hidden forwarding rules before sending.
- Review the sent items and deleted items for scope of damage.
- Notify anyone who received spoofed messages with clear, factual information.
- Contact your email provider or Microsoft to begin blocklist removal.
- Conduct a root cause analysis — how did the attacker gain access in the first place?
For MSPs, having an outbound phishing incident response runbook ready before it happens is the difference between a controlled response and chaos. The reputational impact on the client's domain reputation and customer trust often exceeds the direct technical damage.
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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