Postbox has an excellent memory when it comes to email addresses. When your messages are downloaded, any address that you've sent messages to and received messages from will be added to the Collected Addresses database.
If you're composing a new message and Postbox delivers undesirable auto-complete suggestions, you can take control of this list by removing these Collected Addresses.
First, open the Postbox Address Book by clicking the toolbar button, or by going to either Window > Address Book (on Mac) or Tools > Address Book (on Windows). Then select Collected Addresses on the left side, and delete any addresses you would like Postbox to forget.
To keep Postbox from collecting addresses, go to Preferences > Composition > Addressing and deselect the checkbox for "Add outgoing email addresses."