A: When you send a single email, the To recipient has to be an active contact in Upsales with an email address. If the person isn't a contact yet, click Add new contact in the recipient list to create them. In Copy and Hidden copy you can also type any email address and press Enter.
Email campaigns are sent to contacts in Upsales only, and some of them can be skipped at send time. See Reasons why your emails aren't being delivered.