In Webmail > Compose (Runbox 6 only) you now have the option to save the recipients of a message in Contacts.
This option is found next to the Send button, and if checked, it will add any recipients to Contacts that aren’t already listed there. The new contacts will be saved in a separate Group called “Auto-saved” to make it easier to find them in Contacts.
The next time you compose a message, the saved recipients will show up in the Groups and Contacts list to the right. They will also appear as suggested matches when you start typing in the recipient fields.