I've reworked my configuration and now it works as expected. Here is what it looks like now :
mail_plugins = quota mail_log notify
protocol lda {
mail_plugins = $mail_plugins sieve
}
protocol imap {
mail_plugins = $mail_plugins imap_quota antispam
}
protocol pop3 {
mail_plugins = $mail_plugins
}
Thanks all for your help and for good explanations.
Alexandre.
