I recently installed Icedove in my Debian Desktop and I got an alert box saying “Unable to find specified executable”.
I had preserved the profile from previous installation (Thunderbird) and the add-on Gnome Integration was installed in my older installation which was automatically carried over to my new installation. I do not have Gnome currently installed in my system and I do not have the necessary binary (/usr/bin/notify-send) for the add-on to use.
To solve this, one of the two solutions can be used.
Remove Gnome Integration
If you do not have Gnome installed, it makes more sense to uninstall the Gnome-Integration add-on from Tools>Addons in Icedove/Thunderbird. Restart Thunderbird/Icedove and the alert should not appear anymore.
Install libnotify-bin from package manager
If you are using Debian or similar system, you will find a package called libnotify-bin in Package Manager or install it from terminal.
sudo apt-get install libnotify-bin
Note: if you have not configured your system to allow you to sudo, but you want to be able to, read this article. Otherwise run it as root by using su.
After installing it the alert will probably not appear anymore.