Before you can do this you need to setup your email addresses on your account page. Click on the drop-down arrow at the top right next to your avatar and click "Account Details".
At the bottom on the left, click on the + Email button and enter your email address. This will let the system know that you will be sending emails in from that address.
When you add an email the system will send out an automated email for you to confirm that you own that email address. Follow the instructions in the email.
You can add as many email address as you need. You can now send emails to people.