OWA – ISA server 2006

Could you please advise on the following issue..

1- 3 weeks ago we implemented ISA 2006 on our network. Some email are coming thru and others get message such as ” time delay” Delivery to the following recipients has been delayed.

and everytime we have re-install ISA 2006 in-order to fix OWA.

Is there a easier to fix this problem when or Server restarts??

Regards