You can send up to 50 emails a month without paying anything. If you need to send more than 50 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 "50 emails" mean 50 emails or 50 recipients?
50 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 50 people (1×50=50), 2 messages to up to 25 people (2×25=50), 5 messages to up to 10 people (5×10=50), etc.