What does this have to do with stunnel??
SMTPS uses port 465 conventionally. Using port 25 implies use of starttls, not smtps. Then again if your server is going to instantly demand an ssl certificate a la smtps (which is fairly obsolete now in favour of starttls) then what you've posted seems potentially correct, as you aren't able to provide the details it wants, so it would disconnect. what do you expect to happen?