Hello,
When I send email form mobile phone and log in on PC in cpanel and check roundcube email, there is no email in sent folder. Same situation when I send from PC, there is no email in sent folder on mobile.
why?
Roundcube is a mail client similar to the ones on your phone or PC except it is web based. You should check the settings in your phone and PC mail clients if you think they are not storing sent mail in the correct folder.