How do you send emails in Zendesk?
You compose emails in tickets and business rules send those emails to your customers. Incoming emails from your customers are automatically converted into tickets. If you want to start a conversation with a customer and you don't have a ticket with them already then you will need to create one. For more information on starting proactive email conversations, see the article: Creating a ticket on behalf of a requester.
If you need more information on how email works in Zendesk, see the article: Getting started with email - Part 1: How the email channel works.