RelayBridge

Problem-solving guides

Microsoft 365 SMTP guides for legacy devices

Practical explanations for connecting printers, scanners, multifunction devices, and legacy applications to modern Microsoft 365 mail infrastructure.

Seven starting points

Start with the problem
in front of you.

These guides explain the compatibility boundary and the available solution categories. They are not product installation instructions; RelayBridge remains pre-release.

01Printers and MFPs

Microsoft 365 SMTP for legacy printers

Why older printers and multifunction devices struggle with OAuth, what Exchange Online expects, and where a local bridge fits.

Read guide
02Scan to email

Scan to email with modern authentication

A practical look at the scan-to-email path when the scanner can speak SMTP but cannot authenticate to Microsoft 365 itself.

Read guide
03Legacy applications

Exchange Online SMTP OAuth for legacy applications

How SMTP AUTH, OAuth, XOAUTH2, application identity, and scoped Exchange authorization fit together for older software.

Read guide
04Decision matrix

Microsoft 365 SMTP options compared

Choose between client submission, connector relay, Direct Send, and High Volume Email by identity, recipients, network path, and volume.

Read guide
05Diagnostics

Troubleshooting Exchange Online SMTP errors

Diagnose authentication, Send As, relay attribution, throttling, and TLS failures from the complete server response.

Read guide
06Branch offices

SMTP relay from dynamic-IP branch offices

Understand static-IP, certificate, CGNAT, failover, and port 25 constraints across distributed sites.

Read guide
07TLS diagnostics

Legacy printer TLS handshake failures

Separate STARTTLS mode, TLS versions, cipher suites, certificate trust, device time, and SNI evidence.

Read guide

Scope

Useful now.
Not setup documentation.

The guides describe current Microsoft 365 behavior and RelayBridge’s intended v1.0 boundaries. Final installation, provisioning, and device-configuration documentation will be published only after the release flow is frozen.

Documentation status

Current implementation

See the local bridge architecture.

RelayBridge accepts SMTP from explicitly configured local devices, queues durably, and delivers directly to Exchange Online using OAuth and TLS.

How it works Security