That "backup-mail" icon is neither a F.D.O standard icon, nor provided by any other components (hope I'm wrong). I use lxr.kde.org to search for that icon, no success.
Created attachment 75828 [details]
If no new icon is in the works, I suggest to use "document-export-table".
Git commit 6fd48159f6fc7b7470493d6628321603d9aae237 by Montel Laurent.
Committed on 07/01/2013 at 19:37.
Pushed by mlaurent into branch 'KDE/4.10'.
Fix Bug 311688 - The "Export KMail Data" action uses icon "backup-mail",
which is unavailable
We don't have icons for it. So Use kmail for 4.10
We will ask for new icon
M +1 -1 kmail/kmmainwidget.cpp