It is not possible to designate the sending email address of a customer/member in Core iQ in order to use their main address sometimes and their alternative address other times. Alternative email addresses are mainly used in Core iQ as a means of placing in a 'fall back' email address for when the main item fails. While it is not possible to occasionally send an email to this alternative address, you can reconfigure this information in your core in order to change the alternative address to the main address after the next import. This can be done when the main email address is no longer valid and is bouncing consistently.