The first step is to add and then verify the email address.
First, go to My Account, then click Add/Edit Email addresses:
Then, click on the Add a Verified Email Address button:
Type in the email address you want to add and click Verify. This will send an email to that email address that must be clicked on from that inbox. There is also a verification code that can be entered in manually if you are not the recipient of the verification confirmation – the person can always provide you with that code so that you can verify it for them.
Once the email address is successfully verified, you will see it in the Verified Emails list as well as the drop-down menu for “From” and “Reply-To:” email addresses in the campaign builder:
*Please note that this does not give the Verified Email address owner their own login/password to the account. This only adds the email address so that it can be used as a From or Reply-to email address in a campaign.*