!!! Overview
[{$pagename}] is a [Opportunistic encryption] concept for optimal use of [encryption]. 


Currently implemented in as [StartTLS] and a few other protocols where the he sending server attempts to send a [message] encrypted, but if the receiving server will or can not accept [encrypted] [messages] then the mail is sent [Plaintext]. 
Not all of your [messages] are encrypted this way, but the important takeaway is that it is encrypted whenever possible and should be accepted by all ISPs/inbox providers.


!! [{$pagename}] [SMTP] Submission, [IMAP], [POP], and ManageSieve [clients]
[RFC 7817] describes the [Transport Layer Security] ([TLS]) [server] identity verification procedure for [SMTP] Submission, [IMAP], [POP], and ManageSieve clients.


!! [FTP] 
[File Transfer Protocol] ([FTP]) Securing [FTP] with [TLS] ([RFC 4217]) for negotiating TLS is the character string identified in {TLS-PARM}.  This allows the [client] and [server] to negotiate [TLS] on the control connection without altering the protection of the [data] [Channel].


!! More Information
There might be more information for this subject on one of the following:
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