Can you schedule emails with Postmark?

It is not possible to schedule emails to be sent at a later time through Postmark.

We understand that bulk messages sent through your Broadcast Message Streams, like a Terms of Service Update, may need to be sent out at a later date or time, though scheduling would need to be built-in to your side before the request is made to Postmark.

As for Transactional messages, these are one to one unique messages that the recipient is expecting to receive and are typically triggered by user action. Messages of this sort do not need to be scheduled because they are time sensitive.