Hi,
I have a problem with my email server. When I go to roundcube I see just email from 7 days. Others are not displayed (or erased).
What is the cause ?
Check your mail server, Roundcube wouldn't be deleting your email.
do you know what can be the cause in postfix/dovecot ?
It should not be Postfix, that just transports the email.
Dovecot could be expunging email based on a variety of criteria. It won't do that on its own, the email administrator would have to configure it or setup a cron script or similar.