When I connect from my local mail client (e.g. Thunderbird) the first time (!) with a remote mail server through STARTTLS for a SMTP session then at first a server certificate is delivered. On the Thunderbird email client side a popup appears which prompts me to confirm the remote server certificate. Only AFTER this confirmation I am able to send email through this STARTTLS SMTP connection. Ok. Now lets switch to another scenario where a non-default email client (=NOT Thunderbird) want to send emails through stunnel to the remote mail server. Everything is setup in stunnel.conf and propriatary email client. But how do I get the remote server certificate (for stunnel)? Is there an option in stunnel.conf to receive and auto-accept the delivered server certificate at first? Where is it stored? Can I extract certificate from Thunderbird and import it to stunnel? Ben