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."
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).