Close Mail.
In DT select DEVONthink 3/Install Add-Ons… from the menu.
Select to install the Apple Mail Plugin (v. 4.0.5); install.
Open Mail, Preferences/General/Manage Plug-ins…
Activate the plugin, select Apply and Restart Mail
@cgrunenberg: having installed v. 4.0.5 the “version 4.0.3 is available, you’re using version 4.0.5” problem still persists.