In a recent ISA Server 2006 Level 400 class, we discussed a vulnerability on Exchange server that could lead to remote code execution. The particular remote attack is listed in CVE http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-0213 and rated high in it's severity. If you have customers or run Exchange of all version, check out the article from Microsoft.
This vulnerability and other not so critical ones are described in MSKB 07-026 (http://www.microsoft.com/technet/security/bulletin/ms07-026.mspx). Apply the fixes stated in article immediately, as highest priority.
REMEMBER, PLEASE TAKE THIS MATTER SERIOUSLY, REMOTE CODE EXECUTION=TOTAL CONTROL OF YOUR EXCHANGE BOX. If you run a domain controller on top of that box, the risks are even higher.