https://www.techrepublic.com.com/5208-6239-0.html?forumID=50&threadID=174828
there is the link for the question, which I am duplicating here. Why? Discussions seem to get more attention, but I want to reward the correct answer as well. It’s worth 700 points. Thanks.
We have a customer that we remotely administer that runs MS Exchange server 2000 with service pack 3, on top of Small Business Server 2000. 9 months ago, an intermittent problem surfaced: Email sent to certain Domains got delayed in some of their accounts. It would take 6-15 hours for them to get the notification that the mail was delayed. The issue was fixed by manually installing the post service pack 3 rollup outdates. Just rebooting the server does not solve the problem. All delayed mail then gets sent. That fixed it for 8 months. Now it has happened 3 times this month. We install the latest rollup and it stops,,, for a while.
The installation is vanilla and updated.