Exchange5.5 bad message

I’m running Exchange5.5 sp3 on a NT4.0 sp6 server. Every once in a while I have problems with bad messages, now I have a work around so i can fix it when it happens, but what I am looking for is…What IS a bad message and what makes it accur. I’m looking for the amswer in laymans terms. I can get all the high tech jargon on MStechnet but I would like something easier to understand. Any help would be great