i ve an problem, i ve setting the mailserver as authentification needed for smtp, but when i show about the smtp queue there is a lot of mails definitely spam! how can i see about what's the problem?
You seem to be facing a problem with PHP Mail function SPAM.
I recommend you disable the mail function, better still you can enable logging for PHP Mail function emails sent out of your server.
The issue you will have is that each site that you setup within Plesk can have its own php.ini file. I would recommend to add these two lines to your PHP setting of your website or better yet, add then under your service plan section and do a sync with Plesk.
mail.add_x_header = On
mail.log = /usr/local/psa/var/log/phpmail.log
Once you have added the two lines above, you will need to create the log file in question. Run each of the lines below separately via SSH.