If you still experience issues and errors, please provide MORE informations ( operating system, mail - server in use, Plesk version ( incl. MU ) ... ) and depending errors from your logs.
We really would like to help you, ph_capy, but without further informations, we only can guess what's wrong or what might cause issues/errors on your server. Please have another look on the above mentioned KB - article, and if you can't investigate the issue/cause on your own with the suggestions, please provide informations and depending log - files, as you already see examples at the mentioned KB - article.