How to stop automatic additions to my address book...

[SSonnentag]

Is there an option in a config file somewhere to prevent addresses from being automatically added to my address book when I send someone an e-mail?

[rosali]

I'm not aware of this behavior in default RoundCube. MyRoundCube has an option in user preferences to add recipients of outgoing messages to the addressbook.

[SSonnentag]

I have the latest version and there isn't any such option in the preferences that I can see.  Maybe you have the previous version.