We use a third party from time to time to build emails - we are usually just provided the HTML. When I copy and paste the HTML code into the Marketo email to re-create the email, I am only able to edit the email through the HTML code. Is there a way to make the email editable without having to go into the HTML?
You absolutely can make the email editable within the Marketo email editor, however, it requires a few adjustments to the template HTML for Marketo to understand what to do.
There's some detailed descriptions what syntax is required to help Marketo identify what it should do with sections which can be found here: Email Template Syntax - Marketo Docs - Product Documentation
I'd recommend that if you're getting a third party to build out a template for you that they understand this syntax so you can more easily work with the template they provide within Marketo.
Yes this is possible - Marketo syntax is what gives you editable areas in an email. To enable this functionality, you'll need add the syntax as appropriate throughout the email code. See Email Template Syntax - Marketo Docs - Product Documentation.
Worth noting though that if you have to add this manually every time it's going to likely be more effort than it's worth; I would only look into this if you can ensure it appears permanently in all code supplied from the third party - or ideally to set up a master email template that the majority of your email design will be based around.
Hope that helps.