looks like there is no easy way.
When msg is ready, click 'Save mesage as draft' - this will put it in the Outbox, but with a 'parked' flag On, so it will not be sent when you send other mesages.
Then in the Outbox right click the msg and select
Specials > Remind later > Advanced
Clear all boxes then click OK, then you can define what to happen when (theBat will create a scheduled task to remind you, you can edit it later). But it doesn't seem possible to make scheduler clear the parked flag, so sending can't be automatic - you'll only get a reminder.
Another way could be, put these messages in the outbox of another account which you don't normally use (could be an account with dummy pop3 settings and same smtp settings). Then you can schedule sending from this account, either with theBat scheduler or with Windows one. The disadvantage is you can't schedule sending per message, all pending will be send at once.