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STARTTLS — a protocol command that upgrades an existing plaintext connection to TLS without opening a separate implicit-TLS port.
Email delivery involves several roles and protocols: a client creates a message, servers transfer it with SMTP, and the recipient reads it through IMAP, POP3, or webmail. Transport encryption and sender authentication solve different problems.
STARTTLS — a protocol command that upgrades an existing plaintext connection to TLS without opening a separate implicit-TLS port.
The mechanism participates in message creation, transfer, receipt, or storage. Its role depends on the delivery stage and whether it operates between a client and server or between mail servers.
Verify the server, port, encryption, authentication method, DNS records, and SMTP logs. For delivery errors, record the exact response code and the stage at which it occurred.
