Microsoft has announced a revised deprecation timeline for SMTP AUTH Basic Authentication in Exchange Online, giving organizations an extended runway to modernize legacy email workflows.
The updated schedule reflects customer feedback and adoption challenges, providing clearer milestones through 2027 before the authentication method is permanently retired.
The new deprecation roadmap addresses real-world implementation barriers facing enterprises. Through December 2026, SMTP AUTH Basic Authentication will continue functioning without changes, maintaining current behavior for existing tenants.
At the end of December 2026, Microsoft will turn off the authentication method by default for all existing tenants, though administrators will retain the ability to re-enable it temporarily if business continuity requires.
Research noted after December 2026 will have SMTP AUTH Basic Authentication unavailable by default, with OAuth becoming the sole supported authentication method.
Microsoft plans to announce the final removal date during the second half of 2027, marking the permanent end of basic authentication for client submission endpoints including smtp.office365.com and smtp-legacy.office365.com.
Security Risks Drive Authentication
Basic authentication transmits usernames and passwords as plain text over networks, creating significant security vulnerabilities.
This legacy method exposes organizations to credential theft, phishing attacks, and brute force attempts. The authentication approach also prevents enforcement of multifactor authentication (MFA), leaving email systems vulnerable to unauthorized access.
Microsoft’s multi-year effort to eliminate basic authentication from Exchange Online began in 2019, completing for most protocols in late 2022.
SMTP AUTH remained the sole exception until this planned deprecation. Once disabled, applications attempting basic authentication will receive the error response: “550 5.7.30 Basic authentication is not supported for Client Submission”.
Migration Pathways to Modern Authentication
Organizations capable of implementing OAuth should prioritize migration to this token-based authorization method.
OAuth 2.0 access tokens have limited lifespans and remain specific to designated applications and resources, preventing credential reuse. The protocol enables straightforward MFA enforcement while reducing attack surfaces.
For organizations requiring continued basic authentication functionality, Microsoft offers alternatives including High Volume Email for Microsoft 365 for internal messaging, Azure Communication Services Email for internal and external recipients, or Exchange Server on-premises in hybrid configurations with anonymous relay connectors.
Administrators should inventory current SMTP implementations, identify OAuth-compatible clients, and develop migration plans before the December 2026 default disable date.
The Exchange admin center now includes SMTP AUTH Clients Submission Reports showing whether basic authentication or OAuth is being used, providing visibility into tenant authentication patterns.
The extended timeline balances security imperatives with operational realities, giving customers sufficient time to validate modern authentication alternatives while maintaining Microsoft’s commitment to stronger default security postures across Exchange Online.
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