If you accidentally deleted Postfix log files in Ubuntu (or any other system), you can restore them in the following way:
postfix (en) - B-blog.info
Its not a secret, that DNS BL\RBL lists are the most powerful and effective mechanism of anti-spam defense at this time (2015).
How it works?
IP-address of remote host checked against one or more DNS BL lists (free for all by default). If IP-address is in the list - mail session is rejected.