Mailgun Tutor Needed

Hi there, I need help setting up my Mailgun account to use with [login to view URL] Some specific questions I have are: Why does Mailgun recommend using a subdomain instead of my main domain? How do I know if my DNS settings are set up correctly and if they aren't how do I edit those? Why do they say the $79/mo plan will give you better deliverability. Is this true, or just a marketing ploy? This is the article that explains the way my account needs to be set up for use with [login to view URL]: [login to view URL] Please take a look at the article so you are familiar with what I need to do with Mailgun. I only need to set the Mailgun account up for Close.io. There isn't any other configuration that I need to do besides that. I'm trying to get all this completely set up by Saturday, so let's try to connect soon.