[Dovecot] Populating mailbox dir

Let's say you created a (virtual) mail user but have not created the 

user mail directory. When you first try to access it, the mail directory is created and populated:

ls -a /var/spool/vmail/domain.com/raub/ . dovecot.index.log dovecot-uidvalidity.4d375a39 tmp .. dovecot-uidlist new .Trash cur dovecot-uidvalidity subscriptions

Who populates/creates the initial files and folders in the user mailbox? Dovecot? The mail client accessing it through imap? Elvis? If dovecot, is there a way to customize it a bit, so a few extra folders will also be thrown there?