Can’t send messages to certain addresses

We are running Windows NT 4.0, Exchange 5.5 with SP4, and all client PCs are running either Outlook 98 or 2000.

All of our users are unable to send messages to any addressee with a “direct.ca” or “look.ca” email address. The messages just sit in the outgoing IMS queue and eventually time-out. This problem has been occurring for the last 2 or 3 weeks, and there has been no server configuration changes on our end since the trouble started. There had been no problems with these adresses previously that I am aware of.

Does anyone have any suggestions as to where I can begin to look for a solution???? I have already contacted the folks at Look.ca and they deny they’re having any problems with their email servers.

Any assistance is greatly appreciated.