On Tue, 8 May 2018, DPS Support wrote:
2018.05.08 15:16:07 LOG5[5]: Service [outlook-smtp] accepted connection from 126.10.230.254:52656 2018.05.08 15:16:07 LOG6[5]: failover: round-robin, starting at entry #4 2018.05.08 15:16:07 LOG6[5]: s_connect: connecting 2603:1036:101::2:587 2018.05.08 15:16:07 LOG3[5]: s_connect: connect 2603:1036:101::2:587: Network is unreachable (WSAENETUNREACH) (10051)
[...]
The configured smtp-mail.outlook.com address are resolving to IPv6 addresses as well but the host where stunnel runs does not know how to connect to that address via IPv6. One of the following should fix that:
1) Make your stunnel host IPv6-ready :) 2) Configure the IP address instead of smtp-mail.outlook.com in stunnel.conf 3) Make your stunnel host prefer IPv4 over IPv6 addresses, IIRC gai.conf(5) could help here.
HTH, C.