Unable to send emails to addresses

Hello,

I am running Exchange 5.5 on a Windows 2000 Server. I am experiencing unsuccessful emails messages going out to to certain sites. I have the following message :-

‘Unable to deliver the message due to a communications failure.’

I did not see any other errors on the Event Viewer logs. Have also checked that the e-mail address is valid and I could send to the same e-mail address from another site.

Would appreciate if anyone has any ideas.

Thank you,
Joyce