配置方法：

1）在main.cf里定义如下的smtpd_restriction_classes：
引用:

# 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：
引用:

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：
引用:

foo@extmail.org local_out_only
    bar@extmail.org local_out_only

5）编辑/etc/postfix/local_domains：
引用:

internal.foo.com         OK
    internal.bar.com         OK

6）为3，4，5建立对应的hash文件：
引用:

# 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：
引用:

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：
引用:

554 5.7.1 <bar@extmail.org>: Sender address rejected: Access denied

目前这样的配置就可以比较完整的实现内部用户的功能需求了。其实这种配置的方法不难，关键得分清楚什么阶段，调用什么restrictions即可。