1）在main.cf里定义如下的smtpd_restriction_classes：
# restrictions smtpd_restriction_classes = local_out_only local_in_only local_out_only = check_recipient_access hash:/etc/postfix/local_domains, reject local_in_only = check_sender_access hash:/etc/postfix/local_domains, reject
2）将main.cf里的smtpd_recipient_restrctions定义为：
smtpd_recipient_restrictions = check_sender_access hash:/etc/postfix/local_out_senders check_recipient_access hash:/etc/postfix/local_in_senders permit_mynetworks, permit_sasl_authenticated, reject_non_fqdn_hostname, reject_non_fqdn_sender, reject_non_fqdn_recipient, reject_unauth_destination, reject_unauth_pipelining, reject_invalid_hostname,
3）编辑/etc/postfix/local_in_senders：
引用:
foo@extmail.org local_in_only bar@extmail.org local_in_only
4）编辑/etc/postifx/local_out_senders：
foo@extmail.org local_out_only bar@extmail.org local_out_only
5）编辑/etc/postfix/local_domains：
internal.foo.com OK internal.bar.com OK
6）为3，4，5建立对应的hash文件：
postmap hash:/etc/postfix/local_in_senders
postmap hash:/etc/postfix/local_out_senders
postmap hash:/etc/postfix/local_domains
这样就定义了extmail.org域名里两个用户foo和bar，只允许和internal.foo.com和internel.bar.com 两个内部域的用户来往邮件，对于外部的邮件则没有收或发的能力。如果外部邮件企图给这2个用户发送，则遇到如下错误：
554 5.7.1 <foo@extmail.org>: Recipient address rejected: Access denied
如果这2个内部用户要给外部用户发email，将遇到如下错误：
554 5.7.1 <bar@extmail.org>: Sender address rejected: Access denied