Is it possible to schedule send an email using the default Mail OS app?
Mail doesn't have this feature out of the box but you can use your native Mac apps to schedule an email. My answer is based on macOS Catalina but it should be practically the same besides some minor differences, placement etc.
Using Automator and Calendar:

Create an email workflow using Automator; then
Add the Automator workflow to a new Calendar event (formerly iCal)

1. Create an email workflow using Automator
First open Automator app, which is found in the Utilities folder, and choose New Document. When you see the workflow type chooser, select Application.
Select Mail from the Library pane on the left, and a list of Mail actions will show in the Actions pane to the right. Scroll down and you will see the New Mail Message action. Double-click it or drag it to your workflow.
Once you've composed your message, add the Send Outgoing Mail action to the workflow. Now save the workflow and quit. Make sure the format is set as Application.

2. Add the Automator workflow to a new Calendar event
Open Calendar app (formerly iCal) and choose New from the menubar, or select an existing event to edit.
Give the event a name and set the time and date as per usual. Where we can set an alert, we'll select Custom. From the dropdown, choose Open file, and from the next dropdown, (currently labeled Calendar) select Other. Now browse to the Automator workflow app from the previous step. Save the event and quit.
Should be good to go. Don't forget to keep your Mac turned on.
Edit: Using Calendar without a Automator workflow
The Calendar app lets you send an email as part of an events alert. Similar to the first method, instead of selecting Open file after choosing Custom for the alert type, you can select Email.
Not sure how long this has been an option or if its available in earlier versions of Calendar and iCal.