"Secret" and "Domain" labels in Settings/Email are confusing

I think it would be helpful to have clearer explanations of these terms. 

"Secret" doesn't correspond to any settings in MailGun. I think making the label "API KEY" would be clearer. 

As for "Domain," MailGun has an API Base URL, which I assumed was Satamic's "Domain." It wasn't. So maybe an instruction that this field would normally be the site's domain. 

Thanks!