in main.cf add the line:
smtp_helo_name = myhost.example.com
the default helo is $myhostname, but you can change it to any valid host
name, such as your internet FQDN = Full Qualified Domain Name
oder hier in deutsch lesen:
http://www.pc-experience.de/wbb2/thread,threadid-1859,sid-35cd144b2688a547b2fa6bb3b38753cb.htm
und hier von web.de himself:
http://freemail.web.de/reject/