For me, I expected it to work as for you...
After a lot of trying I finally found something that works:
entry is smtp.xxxx.nl
clicked the box next to it "authentication"
new screen opened
checked the box "execute SMTP authentication (RFC2554)"
checked the box "use specific settings"
then I could enter user/password
Now the entry " smtp.xs4all.nl" has changed to "smtp.xs4all.nl / user = XXXX"
Dunno why this did the trick,but I'm glad my worries are over !