Microsoft has release Service Pack 4 for Exchange Server 5.5. Thanks DosFreak.

Microsoft Exchange Server 5.5 Service Pack 4 (SP4) is a cumulative release that contains all of the updates to Exchange Server 5.5. To download this file from this Web site, follow these instructions:

Create a folder named SP4 on your local hard drive.
Download all of the compressed files listed in the tables below to the SP4 folder.
At the Microsoft Windows NT command prompt, switch to the SP4 folder, type the name of a compressed file, and then press ENTER to display the self-extracting dialog box.
In the Unzip To Folder box of the self-extracting dialog box, type the location of the SP4 folder. Click the Unzip button. Once the self-extraction process is complete, click OK, and then click Close.

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