Some protocols use a protocol-specific command to begin TLS negotiation (STARTTLS for SMTP and POP3), so stunnel needs to know how (and be told) to use that command.

However, I believe you only need to do this if using TLS on a POP3 connection on port 110. POP3S on port 995 should implicitly use TLS.


On Thu, Jan 18, 2018 at 9:51 AM, <peter@easthope.ca> wrote:
P.s.

From:   "Phil Smith III" <phs3stuff@cox.net>
Date:   Wed, 17 Jan 2018 21:59:42 -0500
> protocol = pop3

The stunnel(8) manual has this.
"protocol = PROTO
    application protocol to negotiate TLS
    This option enables initial, protocol-specific negotiation of the TLS encryption."

Is there any need for protocol-specific negotiation?  Why?  Stunnel
provides a tunnel and POP3 works inside.  That's all.  Try the
configuration without the protocol option.

Regards,                         ... Peter E.

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