You can schedule a message in Messages in iOS 16 and iPadOS 16 to send at a later time. Here’s how:
° Launch Mail and create a message.
° Instead of tapping the send icon, tap and hold it.
° You should now see the scheduling menu with four options to either send now, send tonight, send tomorrow morning or send later. Choose the last option.
° You’ll see a Calendar panel that lets you set the date and time for your message to be sent. Just make sure that your iPhone or iPad is actually on at the time you want the message to be send.
I think you mean to schedule an “Email” to be sent at a later date and/or time. The implication that you can send a “message” is misleading. You wouldn’t open “Mail” app, you would open the “Messages” app. In “Messages”, there is no such option that I’m aware of.