Background emails are sent with a web service and require network connectivity. There’s no way to queue those.
If you do not use “Send in background” and go through Mail.app, they can be queued offline. They will go into the Mail.app outbox just as if you composed them in Mail.app offline and send next time you are connected.
There’s really not good ways to reliably queue things for later on iOS and have any reliable expectation of when they would actually get sent.
You could tag those “send later” or something when you are offline, then process them all at once when back online.