Definitely! 📨 When you mark and mail as "Send Later" in Polymail, it will automatically be sent, regardless of whether or not you have a Polymail open on your macOS or iOS devices.
Behind the scenes, when you mark an email as "Send Later" in the app, it gets queued up through your Polymail account and at the time you specify, it will automatically get sent.
Change your mind? Anytime before the time you specify to "Send Later", you can remove the "Send Later" and the email won't be sent.