Found a solution which worked for me:
I changed the build number in the "SystemVersion.plist" (/System/Library/CoreServices/) from 13A598 to 13A603.
After this the manual update worked for me.
Reboot osx86 with the changed build number also works.
P.S.: After rebooting App Store finds the "Mail Update 1.0" and it's possible to install it (again after installation of the downloaded update).
P.P.S. Mail is still workingGo to the full post