How to Send Secure Email in Outlook (The [Encrypt] Subject Line Method)

Need to send something sensitive through Outlook? Microsoft 365 includes built-in email encryption — and there's a quick method that works just by typing in the subject line. Here's exactly how to do it, step by step. Then we'll cover the limitations nobody tells you about.

Method 1: Using [Encrypt] in the Subject Line

This is the fastest way to encrypt an Outlook message. No menus, no settings — just type in the subject line.

You'll see a small lock icon appear on the email, confirming it's encrypted. If the recipient is also on Microsoft 365, they'll see the email normally. If they're on Gmail, Yahoo, or another provider, they'll receive a link to view the message securely through the Office 365 Message Encryption (OME) portal.

Method 2: Using the Encrypt Button

If you prefer clicking over typing, Outlook also has a built-in encrypt button:

Alternatively, go to File → Properties → Security Settings and check "Encrypt message contents and attachments." This method works in older versions of Outlook too.

The Problem With Outlook Encryption

1. It only works smoothly between Microsoft 365 users. If both you and your recipient use Microsoft 365 business accounts, encryption is seamless. But if they're on Gmail, Yahoo, or any other provider, they get sent to a clunky web portal to read your message — and many recipients simply don't click the link.

2. Microsoft holds the encryption keys. Outlook uses Microsoft's encryption, which means Microsoft technically can access your messages. This isn't zero-access encryption — a Microsoft administrator or a court order could potentially unlock your data. If you're sending anything truly sensitive — legal documents, financial information, or personal health data — this is a real risk.

3. Subject lines are NOT encrypted. Even when you use [encrypt] in the subject line, the subject text itself is sent in plain text. Anyone intercepting the email can see what it's about — they just can't read the body. For HIPAA compliance, this means you should never put PHI in the subject line.

4. It's a Microsoft 365 feature — not available in free Outlook. The encrypt button and [encrypt] tag only work with Microsoft 365 subscriptions (Business, Enterprise, or E3/E5). If you're using the free Outlook.com or a basic plan, you don't have this feature.

A Simpler Way: Proton Mail's Built-In Encryption

Here's what makes Proton encrypted email fundamentally different: every email is encrypted by default. No [encrypt] tag needed. No button to click. No hoping the recipient uses the same platform.

With Proton Mail:

• End-to-end encrypted by default — your emails are encrypted on your device before they ever leave. Not even Proton can read them.
• Works with anyone — sending to another Proton user? Seamless, like Outlook-to-Outlook. Sending to a Gmail user? They get a password-protected link that's far simpler than Microsoft's OME portal.
• Zero-access architecture — Proton doesn't hold the keys. Even with a court order, Proton can't decrypt your messages. Microsoft, Google, and Yahoo all hold the keys.
• Subject lines are encrypted too — unlike Outlook, Proton encrypts the entire message including the subject.
• Free to start — 1GB of encrypted email, 150 messages per day, no credit card required.

When to Use Outlook Encryption vs. Proton Mail

The Bottom Line

Yes, you can send secure email in Outlook — type [encrypt] in the subject line or click the Encrypt button. It works. But it's a bandage on a platform that was never designed for privacy.

If you're occasionally sending a confidential message within your Microsoft 365 organization, Outlook encryption is fine. If you care about email privacy — whether for work, healthcare, or personal use — Proton encrypted email gives you real end-to-end encryption without the platform lock-in.