В настоящее время, согласно
RFC 8314, следует использовать неявное шифрование, то есть порты 465/TCP (Submissions), 993/TCP (IMAPS) и 995/TCP (POP3S). В частности в разделе 3 сказано:
Previous standards for the use of email protocols with TLS used the STARTTLS mechanism: [RFC2595], [RFC3207], and [RFC3501]. With STARTTLS, the client establishes a cleartext application session and determines whether to issue a STARTTLS command based on server capabilities and client configuration. If the client issues a STARTTLS command, a TLS handshake follows that can upgrade the connection. Although this mechanism has been deployed, an alternate mechanism where TLS is negotiated immediately at connection start on a separate port (referred to in this document as "Implicit TLS") has been deployed more successfully. To encourage more widespread use of TLS and to also encourage greater consistency regarding how TLS is used, this specification now recommends the use of Implicit TLS for POP, IMAP, SMTP Submission, and all other protocols used between an MUA and an MSP.
Однако следует отметить, что на протяжении примерно 20 лет с конца 90-х использование 465/TCP (aka SMTPS) наоборот считалось устаревшим в пользу 587/TCP (Submission). Но сложившаяся практика и всеобщий тренд на TLS изменили положение вещей.
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