Till now, DMs (Direct Message) had not worked via email. (Not sure if I've tested DMs via notification.)
A site needs interminable configuration in order to serve email! Much of this is due to the plethora of 'defences' which web-mail providers, and others, put up in their interminable battle against spammers. Though, IIRC, this particular problem did not directly have to do with that, it certainly added to the pile of work.
The configuration and testing for this have now been carried out, successfully.
This Task is now Completed.
Task Completed: DM/PM direct messages, via email, now work.
Moderator: omid