I have one machine that had it's hard drive fail 4 times in one month.  I guess that is a good reason for not posting in a while.  I thought this might make up for it.

Some of our customers are still running Exchange 5.5 even though mainstream support for the product will retire this year and extended support will retire next year (http://support.microsoft.com/gp/lifeprode).  So, this may be the last time I get to update this list.  Most of the hotfixes that are listed here are only available by calling Premier Support Services (PSS) at 1-800-936-3500 and asking for the fix. 

All of these fixes require Service Pack 4.

Remember that these fixes include fixes from the past as well, so even if the KB article doesn't mention a incident that may seem like that big of a deal to you, many times there were many other issues before fixed that are very important.  So, I would recommend that you stay up to date.

Newest Admin fix
Fax area codes below 300 may appear as ASCII characters on the Routing tab in Microsoft Exchange Server Administrator
http://support.microsoft.com/Default.aspx?id=821352
03-Jun-2003  01:30  5.5.2657.45  2,510,096  Admin.exe

Newest Performance Optimizer
XADM: "Services List of Dependent Services Could Not Be Enumerated. -800FF336" Error Message When You Try to Use the Exchange Server Performance Optimizer
http://support.microsoft.com/?id=821543
26-Aug-2003  17:58  5.5.2657.63  100,624 Perfwiz.exe

CDO
Meetings are unexpectedly moved one hour ahead or one hour behind in Exchange Server 5.5
http://support.microsoft.com/?id=837010
24-Nov-2004  00:39  5.5.2658.25       802,576  Cdo.dll

Directory (DSAMAIN)
Dsamain.exe Generates an Access Violation and Then Stops Unexpectedly
http://support.microsoft.com/?id=829075
11-Nov-2003  21:44  5.5.2657.78    507,152   Dsamain.exe

Groupwise Connector
XCON: Non-Delivery Report is Generated When You Send a Message to a Novell GroupWise Recipient Who Has a 32-Character Domain Name
http://support.microsoft.com/?id=815228
03-Mar-2003  16:46  5.5.2657.27       400,144  Gwrouter.exe
(Honestly, I choose not to support this myself so I chose the above executable.  Perhaps another would be better.)

Information Store (This fixes a regression in the May 2004 Store Roll-up Fix that was pulled from HERE)
The Store.exe process unexpectedly quits when Exchange Server 5.5 processes a delivery receipt or a read receipt message
http://support.microsoft.com/?id=888621
- Or -
Forwarded message may be discarded as a duplicate in Exchange Server 5.5
http://support.microsoft.com/?id=823799
23-Nov-2004  03:03  5.5.2658.25     2,635,536  Store.exe

Jet
XGEN: -1019 Error Messages When You Try to Recover the Database
http://support.microsoft.com/?id=810579
27-Nov-2002  19:38    5.5.2657.5    807,184    Ese.dll

OWA
MS04-026: Vulnerability in Exchange Server 5.5 Outlook Web Access could allow cross-site scripting and spoofing attacks
http://support.microsoft.com/?id=842436

IMC
Public
MS03-046: Vulnerability in Exchange Server Could Allow Arbitrary Code Execution
http://support.microsoft.com/?id=829436
MSEXCIMC.exe 5.5.2657.72

I expect that there will be another IMC fix that will come out in the near future.  If I see it I will post it.

MAPI
Custom MAPI program quits unexpectedly with an error in Emsmdb32.dll
http://support.microsoft.com/?id=821661
30-Jul-2003  04:59  5.5.2657.57  565,008  Emsmdb32.dll

Mailbox Cleanup Agent
DM: Exchange Server 5.5 Post-Service Pack 4 Mailbox Cleanup Agent Fixes Available
http://support.microsoft.com/?id=309010
Mbclean.exe is incremented to version 5.5.2655.34

Migration Wizard
XFOR: Migration Wizard Does Not Migrate Reoccurring Meetings from Lotus Notes R5
http://support.microsoft.com/?id=274593
Mlmignts.dll 5.5.2654.20

MTA
The Message Transfer Agent service continues to accept messages after the hard disk that contains the MTADATA folder fails in Exchange Server 5.5
http://support.microsoft.com/?id=839138
20-Apr-2004  18:08  5.5.2658.6      1,942,800  Emsmta.exe

I hope that this helps you all.