I am seeking for advise as to how to control the time setting of the mail notification, more specifically about the default notification of 'Downed Device Detection'. because of some update maintenance, our server is supposed to be down during a certain period daily. we do not want to receive email at that specific time everyday, can anyone please suggest if there is any settings to be done regarding that?

Any advise is highly appreciated.

Look into the 'Maint' plugin. It allows you to specify maintenance windows where alerts will be suppressed.

Also intropage plugin is a dashboard that will show you these "maintenance tasks" among other things.

So that could also help you give other people visibility on what's going on.