Postpone Sending Email making use of Mail.app
I think that you do not take into consideration Deferrersender (web based) to be suitable. It's not, yet it supplies.
UPGRADE : A choice, that does not make use of Mail.app (yet Thunderbird ) would certainly be to make use of SendLater. An expansion that does specifically that. Yet it's for Thunderbird. I could not locate a comparable plug-in for Mail.app.