[stunnel-users] How to get the remote mail server certificate before/at first connect?

[Ben Stover]

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