Hi,

Port 465 is typically for implicit-SSL, whereas port 587 is for explicit-SSL (ie: requires the client to issue a STARTTLS command after connecting).

I'm not sure exactly what stunnel config changes are needed to deal with that different protocol, but that is where you shoukd start looking.

"protocol = smtp" needs to be added to the stunnel.conf section in order to enable the explicit (STARTTLS) SMTP negotiation.