Microsoft 365 OAuth Email Integration Standard

Designed a secure, vendor-agnostic Microsoft Graph email integration standard for third-party SaaS applications using Microsoft Entra ID and Exchange Online.

Problem

Modern SaaS applications increasingly require OAuth-based integration with Microsoft 365 to send email on behalf of an organisation. A consistent implementation standard was needed to replace legacy SMTP authentication, reduce security risks associated with broad mailbox access, and provide a repeatable deployment model for future application integrations.

Solution

Designed a reusable Microsoft 365 email integration standard based on OAuth 2.0, Microsoft Graph and Exchange Online.

The solution establishes a dedicated Microsoft Entra App Registration for each application, uses Microsoft Graph Mail.Send application permissions, and implements Exchange Online Application RBAC to restrict mailbox access to approved Shared Mailboxes. Operational guidance was documented through a production-ready implementation runbook covering deployment, security, validation, monitoring and ongoing maintenance.

Architecture

Third-Party Application >

Microsoft Entra ID (OAuth 2.0 App Registration) >

Microsoft Graph SendMail API >

Exchange Online >

Approved Shared Mailboxes (Application RBAC Scoped) >

Recipients

Outcome

Established a repeatable and security-focused integration pattern for Microsoft 365 email-enabled applications. The design standardises future SaaS onboarding, promotes least-privilege access through Exchange Online Application RBAC, improves operational consistency, and provides comprehensive auditing using Exchange Message Trace, Microsoft Entra Sign-in Logs and Mailbox Audit Logs.

Key Takeaways

Implemented Microsoft Graph OAuth authentication as the preferred integration pattern for Exchange Online email services.
Applied Exchange Online Application RBAC to restrict application access to approved Shared Mailboxes and reduce the attack surface.
Developed a reusable operational runbook to standardise future Microsoft 365 email integrations.
Built the solution around least-privilege principles, operational monitoring and long-term maintainability.