/usr/share/kde4/config/emaildefaults is in plasma-active-default-settings 1:0.4-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | [Defaults]
Profile=Default
[PROFILE_Default]
EmailClient=kmail-mobile -t %t -s %s -c %c -b %b -B %B -A %A
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