Problem sending mail with emailer Errno::ECONNRESET (Connection reset by peer)

My mail is ready to be sent .. (can see it in the log..)
but I cannot find why I get this error.. please help welcome !
I am working in stand alone dev on Mac OX 10.4.11(Tiger) and running
Postfiw on locahost....

here is the log :

/usr/local/lib/ruby/1.8/net/protocol.rb:133:in `sysread'
/usr/local/lib/ruby/1.8/net/protocol.rb:133:in `rbuf_fill'
/usr/local/lib/ruby/1.8/timeout.rb:56:in `timeout'
/usr/local/lib/ruby/1.8/timeout.rb:76:in `timeout'
/usr/local/lib/ruby/1.8/net/protocol.rb:132:in `rbuf_fill'
/usr/local/lib/ruby/1.8/net/protocol.rb:116:in `readuntil'
/usr/local/lib/ruby/1.8/net/protocol.rb:126:in `readline'
/usr/local/lib/ruby/1.8/net/smtp.rb:664:in `recv_response'
/usr/local/lib/ruby/1.8/net/smtp.rb:651:in `getok'
/usr/local/lib/ruby/1.8/net/smtp.rb:686:in `critical'
/usr/local/lib/ruby/1.8/net/smtp.rb:649:in `getok'
/usr/local/lib/ruby/1.8/net/smtp.rb:631:in `mailfrom'
/usr/local/lib/ruby/1.8/net/smtp.rb:544:in `send0'
/usr/local/lib/ruby/1.8/net/smtp.rb:472:in `sendmail'
/usr/local/lib/ruby/gems/1.8/gems/actionmailer-2.0.1/lib/action_mailer/
base.rb:588:in `perform_delivery_smtp'
/usr/local/lib/ruby/1.8/net/smtp.rb:379:in `start'
/usr/local/lib/ruby/1.8/net/smtp.rb:316:in `start'
/usr/local/lib/ruby/gems/1.8/gems/actionmailer-2.0.1/lib/action_mailer/
base.rb:586:in `perform_delivery_smtp'
/usr/local/lib/ruby/gems/1.8/gems/actionmailer-2.0.1/lib/action_mailer/
base.rb:469:in `__send__'
/usr/local/lib/ruby/gems/1.8/gems/actionmailer-2.0.1/lib/action_mailer/
base.rb:469:in `deliver!'
/usr/local/lib/ruby/gems/1.8/gems/actionmailer-2.0.1/lib/action_mailer/
base.rb:352:in `method_missing'
/Users/yves/Sites/presdemoi.net/TRY-yves-1.0/app/controllers/
users_controller.rb:343:in `create'

here is my coding in the controller :

UserNotifier.deliver_login_key_notification(@user)

is that a Postfix config problème.. ?