It's always recommended to check your emails actually look like before sending them to hundreds of prospects.
When editing an email step in a campaign, you can click on the Send Preview button to send a preview email:
You can choose who you want to send the email to and what sending address to use.
You can also choose to fill the templates variables in your email with values from the last created prospect. This is useful to test if your variables and fallbacks are correctly set up.