The Exchange Server 5.5 product lifecycle ends on December 31, 2005.
You might want to look at a GroupWise migration now. The process is documented _here_.
And since we're on the topic, if you're running Exchange5.5, you might want to look at upgrading your Server NOS from NT Server 4.0 to OES.
From CERT.Org's July 2005 documented vulnerabilities:
Or, if you must, GroupWise works on a Windows Server too!
open enterprise servernovellgroupwisenetwaresecurity