When running xboard 4.2.7 in cmail mode, the error message "Failed to invoke cmail" is produced whenever File | Mail Move is selected. The fix for this is documented at http://lists.gnu.org/archive/html/bug-xboard/2004-02/msg00019.html. I have applied this fix and can confirm that it works.
Created attachment 86076 [details, diff] xboard-4.2.7-cmail.patch Here's the patch.
In portage. Thanks for the bug report and patch.