iMessage to iCloud email
It will appear on all devices switched on &amp; signed into iMessage, that have that email address set as a contactable destination for iMessages.
It is under their control rather than yours.

If, for instance, they have their phone with them but are outside a data reception area [middle of the countryside etc] yet also have their Mac at home running &amp; signed in, it will arrive at the Mac &amp; be shown to you as Delivered [&amp; even as Read if they left Messages open with your view-pane frontmost] - yet they may not actually see it until they get home.