Reason : You have not set/created your Identity in roundcube
1. Login to your Round Cube webmail interface.
2. Click on Settings Tab.
3. Click on Identities Tab.
4. Create your own identity from New Identity Tab.
5. Make sure that you're tying the correct email address in "E-Mail" Box. Click on Save.
6. Then go to the emails and try to create a new message and you'll find your displayname <firstname.lastname@example.org> in Sender Box.