Your app sends to localhost.
Relayway handles Microsoft 365.
Relayway is a Melosso open source SMTP relay for Microsoft 365. Send to localhost:2525 and Relayway forwards via OAuth and the Microsoft Graph API.
Invisible by design
Relayway runs between the application and Microsoft 365. Changing the SMTP host is the only application change.
Drop-in SMTP server
Applications send to localhost:2525 without authentication. Relayway forwards each message to Microsoft 365.
Microsoft Graph forwarding
OAuth tokens are acquired and renewed by Relayway. Applications hold no tokens or credentials.
Local-only by design
Accepts connections from localhost only. The relay has no network exposure.
Cross-platform
Runs on Windows, Linux, and Docker. Configuration via JSON file or environment variables.
Why it exists
Microsoft deprecated Basic Auth for SMTP in Exchange Online. Applications that relied on smtp.office365.com:587 with a username and password suddenly needed to speak OAuth. Relayway bridges that gap: it accepts old-style SMTP locally and handles the OAuth handshake with Microsoft Graph so you don't have to touch your application code.
Relayway: SMTP Relay for Microsoft Graph
A local SMTP relay that bridges legacy applications to Microsoft Graph after Microsoft removed basic authentication from Exchange Online.
License and availability
Relayway is free and open source, released under the AGPL-3.0 license. It runs on Windows and Linux and Docker, and the full source code is available on GitHub. Use it, adapt it, and share it; there are no usage tiers and no features held back.