When sending an email to a single recipient from the deals tab, select the contact you would like to email by clicking on the rolodex icon under the People, paging through the people tied to the deal using the < > in the window that appears, then click on the contact’s email address. If you have multiple deals with a given contact, emailing from the deal level is a huge advantage because the email will be logged as an activity within the selected deal.
When sending emails to multiple contacts, filter your deals to just the ones you wish to reach out to, check the box at the top of the column of boxes on the left to select all deals that fit that filter, then click Send Campaign. This will email the primary contact on each of the deals selected.