Inspect Application Permissions: Some applications or APIs require specific app passwords or registered permissions to access the SMTP relay. Review Firewall Rules: Ensure that outbound traffic on port 587 is not being blocked by network security groups or local firewalls.
Optimizing Microsoft 365 SMTP Queue Management for Reliable Delivery
Verify Port Settings: Ensure that the port is set to 587 and that the connection type is set to TLS or STARTTLS. This address directs your traffic to the Microsoft infrastructure responsible for relaying your messages.
1 Client was not authenticated" error, which occurs when the server requires encryption, but the client is attempting to send data in plain text. Alternatively, port 25 is often used for server-to-server relay between mail servers, although many cloud providers restrict its use for third-party applications to combat spam.
Optimizing Microsoft 365 SMTP Queue Management for Better Delivery
Another frequent issue is the "550 5. A common error is the "535 5.
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