Hi partner,
Yes, you could use D365 workflow to send email automatically with your requirement.
Go to Settings->Process and create a new workflow.

In entity list, select the entity in which you want to trigger the auto-send function.
Then we should set the trigger conditions and actions.
According to your description, you said that you want to send email when you post a new order, so we could set the condition as when a record is created.

Selet "send email" as your action. Click "set properties" and design your email content.

Finally save and active this workflow.
And another import thing is that if you want to send/receive emails in your instance, you should configure the exchange server in your instance first.
You could follow the steps in the below doc.
https://docs.microsoft.com/en-us/power-platform/admin/connect-exchange-online
Hope it helps.
Best Regards,
Leo