The address book is used to populate that drop-down list, so make sure the old address is deleted from there. Also, when in the Address book, look for "Collected addresses" in the left pane. The Appsuite software stores the addresses of emails you send out as well as receive. So delete it from there also. You can control this by clicking on the gear icon in the upper right corner of the screen on the Inbox page, then on Settings>>>>>>Mail>>>>>>>Behavior. You will see the boxes to check there for incoming and outgoing messages.