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.