Runs locally
Your mail does not pass through a RelayBridge-operated cloud service.
Open-source•Local-first•Microsoft 365
Modern email for legacy devices.
RelayBridge starts with Microsoft 365, letting printers, scanners, and legacy applications use modern authentication without routing mail through another relay provider.
MAIL PATH / LOCAL-FIRST
Runs locallyOpen sourceCertificate identityDurable queueNo relay cloud
The compatibility gap
Your legacy devices and applications speak SMTP.
Microsoft 365 expects modern authentication.
Many reliable devices and applications cannot implement modern Microsoft authentication. RelayBridge gives them a local SMTP endpoint while handling the modern connection to Microsoft 365—because replacing working hardware or using another hosted relay should not be the only choices.
How it works
A short local hop.
A modern Microsoft connection.
- 01
Device submits locally
A printer, scanner, or application sends ordinary SMTP to RelayBridge on the trusted local network.
- 02
RelayBridge protects and queues
RelayBridge restricts the device, accepts the message safely, and persists it in a durable local queue.
- 03
Microsoft 365 accepts it
RelayBridge uses its certificate identity, TLS, and OAuth to submit the original message to Exchange Online.
Why RelayBridge
Built around control,
not another cloud.
A focused bridge for a specific infrastructure problem, with the mail path and security model kept visible.
Open source
RelayBridge source is public under the Mozilla Public License 2.0.
Certificate authentication
Certificate-based OAuth replaces password-based Microsoft authentication.
Scoped permissions
Exchange Application RBAC is designed to restrict RelayBridge to an authorized sender.
Durable delivery queue
Accepted messages are queued locally and temporary failures are retried.
Made for existing equipment
Devices can continue using ordinary SMTP on a trusted local network.
Local-first by design
Your relay.
Your network.
Your mail.
Messages go from your device to your RelayBridge instance and then directly to Microsoft 365.
No RelayBridge accountNo product account is required for the mail path.
No RelayBridge cloud mailboxRelayBridge does not receive or hold your mail in a hosted mailbox.
No hosted relay dependencyDelivery does not require a third-party SMTP relay subscription.
Open source
A security model
you can inspect.
RelayBridge source is public on GitHub under MPL-2.0. An unsigned Windows release candidate is available for evaluation and community testing; a signed stable release is not yet available.
Public release candidate