I don't use imap, as on my HTC Sensation (Android), it has some issues :-) I use POP3 account. I am not sure what RoundCube uses, but I simply did not like usage of email form, which is created, when user account is added. E.g. my user account is called pekr, my nick name, so when I set-up such an user, default email created by system is
pekr@domain.name. So I deleted it, and entered
firstname.lastname@domain.name. So - actually you are correct - email name is being used, but I did not like it's form as provided when adding user accounts. Easy enough to adapt though ...