If you are referring to the From/Reply-to options or changing the main Contact on file, any email address you'd like to use must be verified in the account beforehand. Please see steps on how to verify an email address in this short video by clicking here.
After an email address has been verified, you will find the address available to choose from in the dropdown menu in the header of your draft when selecting a From/Reply to email address. In addition, verified email addresses will be available to choose as a Contact address in the My Account page under User Email. See steps on how to change the main Contact email address here: Update Account Contact Email Address
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