Check the connection in order
A practical checklist for common connection and authentication errors. Test DNS, outbound TCP port 587, STARTTLS, authentication, sender address, and finally the generated credentials.
Read the failure by stage
A timeout before the SMTP greeting points to DNS, routing, or a firewall. A TLS error points to device time, certificate support, or an old TLS stack. An authentication rejection points to copied credentials or a revoked connection.
If authentication succeeds but sending fails, verify that the envelope From address exactly matches the connected mailbox. Change one variable at a time and keep the exact SMTP status code, but never the password or message body.
Use a controlled test path
Resolve the hostname from the same network as the application, then test that TCP 587 opens. The SMTP banner should arrive before authentication; this separates network problems from credential problems.
After STARTTLS, retry with a newly generated connection when credentials are uncertain. A successful send should increase that client’s last-hour count. If it does not, record the timestamp and SMTP status for investigation without including secrets.
Before you save
Use the connected Microsoft 365 address as the sender. Store the generated password securely; it cannot be displayed again.
Send a small test message to an address you control. Confirm delivery before enabling production notifications.
If the test fails
Check that port 587 is permitted by the network, STARTTLS is enabled, and no spaces were copied into the credentials.
Never share passwords or message content. Revoke and recreate the connection if the password is lost.