If an unwanted address is auto-completing in the Compose window, the solution is to remove it from the Address Book(s) Postbox is using.
- Address a message with the contact. Allow the wrong address to auto-complete.
- Right-click the address and select Open in Address Book.
- Delete the contact.
If you cannot locate the contact using the above method, then please open the Postbox Address Book and search for the contact you're trying to locate. Be sure to search through your Personal Address Book but also the Collected Addresses as well.