You can send up to 150 emails a month without paying anything. If you need to send more than 150 emails a month, then you'll want to sign up for a paid Direct Mail account. There are a variety of pricing plans available to fit your budget.
To sign up, choose Direct Mail > Direct Mail Account from the menu bar.
Does “150 emails” mean 150 emails or 150 recipients?
150 emails. When Direct Mail sends your message, each recipient on your list gets their own, distinct copy of the message, each of which counts as one email.
If you wanted to remain on Direct Mail's free pricing plan, you could send 1 message to up to 150 people (1×150=150), 2 messages to up to 75 people (2×75=150), 3 messages to up to 50 people (3×50=150), etc.