Microsoft Basic Auth Deprecation 2026: Cold Email Migration Guide
By Puzzle Inbox Team · July 16, 2026 · 9 min read
Microsoft basic auth deprecation 2026. Complete migration guide for cold email teams. OAuth setup, sending platform compatibility, pre-warmed M365.
Microsoft Basic Auth Deprecation 2026
Microsoft deprecated basic authentication for SMTP and IMAP in 2024-2025. By 2026, all Microsoft 365 cold email connections require OAuth 2.0. Cold email teams using app passwords have been migrating or face outages.
What Changed
Basic Auth (Deprecated)
- Username + password for SMTP/IMAP
- App passwords for MFA
- Permanent password storage
- Security vulnerabilities
OAuth 2.0 (Required)
- Token-based authentication
- Refresh tokens (auto-renewal)
- Granular permissions
- Better security model
Timeline
- October 2024: New tenants disabled basic auth
- April 2025: Existing tenants required migration
- October 2025: Basic auth deprecated for SMTP
- 2026: OAuth-only
OAuth Setup Process
Step 1: M365 Admin Settings
- M365 Admin Center → Settings → Modern Authentication
- Enable Modern Authentication
- Allow third-party OAuth apps
Step 2: Connect Sending Platform
- Smartlead: OAuth flow native
- Instantly: OAuth popup
- Lemlist: OAuth supported
- Reply.io: OAuth supported
Step 3: Verify
- Send test email
- Check SPF/DKIM/DMARC pass
- Monitor 24 hours
Common Migration Issues
OAuth Permission Denied
Admin enable third-party OAuth apps.
MFA Conflicts
Use Authenticator app, not SMS.
Token Refresh Failures
Re-authenticate if persistent.
Pre-Warmed M365 with OAuth
Pre-warmed M365 from Puzzle Inbox arrives OAuth-ready:
- Real M365 tenants
- OAuth pre-configured
- 5-minute connection
- Pre-warmed 4-12 weeks
- Replacement included
Microsoft basic auth deprecation is final in 2026. Migrate to OAuth or face outages. Pre-warmed M365 inboxes from Puzzle Inbox arrive OAuth-ready.