Upon entering an email address into a custom field located on the Account Page, it is not selectable in the Send Email Page under Recipients in CRM Online.
An error occurs if there are no other email fields present on the Account Page: No Email addresses defined for [Account Name].
The Approved Email functionality queries only those fields which have the type of Email on the Account Page. The type of the custom field must be Email.
CRM Help Documentation: Components of Approved Email