[Dovecot] Drafts, Trash, Sent

Hi, I'm testing a dovecot installation using thunderbird as imap client. The first user I configured has regolar folders: Drafts, Sent and Trash, and thunderbirs shows each one with its icon. The second user shows these folders with a standard icon, and I'm not able to save email in sent.

I cann't understand why there is this difference, and how dovecot manage these folder. Can anyone help me?

Thanks a lot Roger