I installed the addon TbSync (https://addons.thunderbird.net/en-US/thunderbird/addon/tbsync/) to sync my address book to my email provider. After installing I added a new address book that is being synchronized. I also set this address book as the default startup directory. Now when I have an email open and I right click the email address of the sender to add it to my address book, it adds it to my "Personal Address Book". This one is not synchronized and I don't want to add it to this address book, I want it to be added to my Posteo (name of my email provider) address book, which is synchronized. I also set the option "Automatically add outgoing e-mail addresses to my" (in Preferences -> Composition -> Addressing) to the Posteo address book. But the behavior described above stays the same. It would be very nice if I could add it directly to my Posteo address book, so that I don't have to move it there or manually put it there.