It's probably already the default account in Mail. Don't do anything else with that all for now - I don't use CraigsList, but for you, I looked up a personal in my browser and hit the email link. It pulled up in the Gmail app, which is no surprise as that app is my default association for email links. I'd bet on the ad I tested with that all you need to do is change your default association. Easy:
From the home screen, select the three-lines settings button, then hit "Settings". Find "App Associations", then change "Email - Compose (Gmail)" to "Mail". Try again and let me know.
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