* coming from servers a server which does not understand "temporary failure" SMTP codes and does (i.e. not resend emailsRFC-conformant)* coming from non-existent senders (i.e., back sender validity verification is performed)* listed in some RDNSBL lists (see below) <!-- Do not change the below line!!! -->== dnsbl ==We use the following realtime DNS blacklists:* bl.spamcop.net (see http://www.spamcop.net/)* sbl.spamhaus.org (see http://www.spamhaus.org/)* cbl.abuseat.org (http://www.cbl.abuseat.org/) If your mail server address is in that lists, your mails will be rejected.