IIS 5.0 SMTP Server => Connection dropped by remote host

Guys,

Can help me please, i had a problem with the IIS 5.0 SMTP server...

Each time when i use outlook to send an email using that smtp server locally, The email is sent under outlook express.. but then when i go to the SMTP Queue, those emails are still there...

Later on, in the Event Log stating:-

Event ID 4000 :
Message delivery to the remote domain "yahoo.com" failed for the following reason. The connection was dropped by the remote host.


Event ID 4000 :
Message delivery to the remote domain "hotmail.com" failed for the following reason. The connection was dropped by the remote host.



May i know what causes that? and why my emails are not sent out and keep on stuck in the Queue folder!!@ Thank You