Roundcube has some features that make use of contacts users collected in their addressbook(s). However, the process of adding new contacts was mostly manual and slow. Here come new ways to automatically collect contacts while using the webmail.
Automatic collection of email addresses of recipients that were used in a sent mail is quite a common feature of a webmail application. There were some plugins available already, but starting from Roundcube 1.5 this becomes a core feature.
You’ll have also a separate addressbook for trusted senders. These are email addresses you collect when allowing remote resources to HTML emails. This contacts source will also be used when deciding that a read receipt should be send automatically, or not.
Both addressbooks are configurable, so you can set them to point to any other writable addressbook, e.g. your main addressbook. You can also disable this functionality. Read more about the change in this ticket.