OAuth SMTP for Cold Email in 2026: Step-by-Step Setup
By Puzzle Inbox Team · July 17, 2026 · 9 min read
OAuth SMTP for cold email in 2026. Step-by-step setup for Google Workspace and Microsoft 365. Why OAuth replaced app passwords.
OAuth SMTP for Cold Email in 2026
OAuth 2.0 has replaced basic authentication for cold email SMTP connections. Google deprecated app passwords for sending in 2022. Microsoft followed in 2024-2025.
What OAuth SMTP Is
OAuth 2.0 is token-based authentication. Instead of storing passwords, OAuth uses access tokens (short-lived) and refresh tokens (longer-lived).
Why OAuth Won
Security
- No password storage
- Granular permissions
- Token revocation easy
UX
- One-time setup
- Auto-refresh tokens
- Provider-controlled access
OAuth Setup for Google Workspace
- Add new inbox in sending platform
- Select Google Workspace
- Click "Connect with Google"
- OAuth popup opens
- Log in with Workspace account
- Grant permissions
- Connection verified
OAuth Setup for Microsoft 365
- Verify Modern Auth enabled in admin
- Add Microsoft inbox in platform
- OAuth popup to Microsoft login
- Complete MFA if required
- Accept permissions
- Connection verified
Common Issues
- "Admin Approval Required" — admin whitelists app
- MFA loop — use Authenticator app
- Token expiration — re-authenticate
- Permission denied — disconnect and reconnect
OAuth vs App Passwords
| Feature | OAuth | App Password |
|---|---|---|
| Available 2026 | Yes | Deprecated |
| Security | High | Lower |
| Permissions | Granular | Full account |
| Status | Required | Deprecated |
Pre-Warmed Inboxes and OAuth
Pre-warmed inboxes from Puzzle Inbox arrive OAuth-ready:
- Workspace or M365 pre-configured
- Modern Auth enabled
- 5-minute OAuth connection
OAuth SMTP is the only option for cold email in 2026. Setup takes 2-3 minutes. Pre-warmed inboxes from Puzzle Inbox arrive OAuth-ready.