Meine frage ist folgende:

Wie kann ich Postfix beibringen das er keine Mails von nicht existierenden Benutzern annehmen soll.

du hast dich unklar ausgedrückt, willst du nicht existierende locale oder "Absenderbenutzer"?

# REJECTING MAIL FOR UNKNOWN LOCAL USERS

#

# The local_recipient_maps parameter specifies optional lookup tables

# with all names or addresses of users that are local with respect

# to $mydestination and $inet_interfaces.

#

local_recipient_maps = unix:passwd.byname $alias_maps

#local_recipient_maps = proxy:unix:passwd.byname $alias_maps

#local_recipient_maps =

# The unknown_local_recipient_reject_code specifies the SMTP server

# response code when a recipient domain matches $mydestination or

# $inet_interfaces, while $local_recipient_maps is non-empty and the

# recipient address or address local-part is not found.

#

# The default setting is 550 (reject mail) but it is safer to start

# with 450 (try again later) until you are certain that your

# local_recipient_maps settings are OK.

#

#unknown_local_recipient_reject_code = 550

unknown_local_recipient_reject_code = 450

Guck dir da mal an ist sehr hilfreich

http://www.pl-forum.de/t_netzwerk/postfix-UCE-HOWTO-de.html