Currently all emails from our company on netsuite are sent from/reply to our orders@ email, would like to make it so PURCHASE ORDERS specifically send from/reply to a different email …how can I adjust this?

Not sure how you are triggering the email, but if you use a workflow with a Send Email Action then you can customize the Sender, Subject Line and Body of the email

We have a send email check box on the transactions that is auto checked and triggering the email. Currently the email preference is the orders@ , hmmm I may try to create a workflow for these specific transactions. The send email isn’t working for credit memos so that made me think maybe a workflow would be suitable for CMs and POs to trigger the email and from address?

Yes I usually like to turn OFF the default Send Email checkbox for transactions and use WF's to control all that. Way more flexibility and control availalbe instead of having to rely on the standard behavior which can be somewhat limiting

I’m creating this workflow for send email, how do I customize the sender / subject line and body ? This is my first go , so I am very ignorant of how to do these.

Under the parameters of your Send Email action you can define the sender and receipient