Editors' Note: According to information provided by Apple at http://support.apple.com/kb/TS2486, "If Mail Update 1.0 does not appear in Software Update, you do not need to install it."
Install the updates to your mail client, to patch it up for work with latest version of Mac OS.
What's new in this version:
This update addresses stability issues with Mail.
After updating to Mac OS X 10.5.6, Mail may unexpectedly quit. This can be caused by using a copy of Mail that wasn't updated properly (you may need to install the Mail Update).