How to Assign an Email Address to a Sequence (Pro Account)

By default, all emails are sent out from your default email address:

The default email address for the entire account is set under Inboxes.

By adding additional email accounts (Pro only), you can set sequences to send from any of the email addresses you added. 

From the Sequence ->  List page, click the sequence you want to change the email account to send out from.
Then under "Action" click "Settings": 

Here you can set which email address to use to send emails for that sequence:

Changes are saved automatically and you can go back to Sequence -> List page to verify if the change you made has been applied. 

Note: If a sequence already started on prospects before you've changed the email account to send emails from, the follow-ups will still be sent out from the email account that's previously used to send the emails for that sequence. 
If you want to change the email address to send the follow-ups, you can remove the old email address from your Inbox settings.