Thunderbird automatically adds outgoing email addresses that are not in any address book to the 'Collected addresses' address book or any other address book you select. This is where you can switch this off or choose a different address book to collect those email address. via 'Menu bar' 'Tools' > 'Options' > 'Composition' > 'Addressing' tab via 'Mail Toolbar' 'Menu icon' > 'Options' > 'Options' > 'Composition' > 'Addressing' tab To stop collecting: uncheck 'Automatically add outgoing email addresses to my:' To collect email addresses but change the address book : Select: 'Automatically add outgoing email addresses to my:' and then chose an address book from the selection offered in the drop down located to the right.