By default, the email sender on your email campaign is the user's email who set up the campaign. We've given you the ability to override the sender on your email campaigns.
To change this, make sure you're in your email campaign under Email Marketing > Email Campaigns > Your Email Campaign. In the top right corner of the campaign, click the "Sending User" dropdown to change the sender of the email.
Note: This option won't appear if you only have 1 user in your account. In that case, it defaults to the main user as the sending user.