Leaving message on IMAP account

[Bastien]

Hi all,

i setup an IMAP account in mail-sources like this :

,----
| (setq mail-sources '((file :path "/var/spool/mail/me")
|                    (imap :server "ser.ver"
|                          :user "account"
|                          :password "******"
|                          :port "143")))
`----

All mails are retrieven from the server, but i don't know how to
*leave* them on it (for access by another machine).  I read the
manual, but can't find help on this - surely very common - topic.

I also would like to store answers on the IMAP server : is it
possible ?

Thanks for any hints,

-- 
Bastien