Before the greeting
A timeout or refusal before an SMTP banner points to DNS, routing, firewall, port, or service reachability.
During STARTTLS
Check device time, DNS hostname use, firmware, TLS support, and certificate validation. Do not fix negotiation by disabling transport security.
During authentication
Confirm STARTTLS is active, the generated smtp_ username is used, the one-time password has no copied spaces, and the connection is not revoked.
After authentication
Confirm the envelope sender matches the connected mailbox and check message size and recipient count.