Postfix邮件系统的配置主要通过main.cf文件，路径在/etc/postfix下。如果要允许其他服务器使用Postfix的服务，需要重点关注inet_interfaces和mynetworks这两个参数。

# The inet_interfaces parameter specifies the network interface
# addresses that this mail system receives mail on. By default,
# the software claims all active interfaces on the machine. The
# parameter also controls delivery of mail to user@[ip.address].
#
# See also the proxy_interfaces parameter, for network addresses that
# are forwarded to us via a proxy or network address translator.
#
# Note: you need to stop/start Postfix when this parameter changes.
#
inet_interfaces = all

要注意的是，修改这个参数必须要停止再启动Postfix，单纯的restart是不生效的。

# You can also specify the absolute pathname of a pattern file instead
# of listing the patterns here. Specify type:table for table-based lookups
# (the value on the table right-hand side is not used).
#
mynetworks = 11.12.13.14,127.0.0.0/8

这个参数指定了要使用Postfix的IP地址，如果不设定正确，会在maillog里看到类似如下的提示信息：

Dec 1 11:16:21 mail postfix/smtpd[21197]:warning: 11.12.13.14: address not listed for hostname banping.com
Dec 1 11:16:21 mail postfix/smtpd[21197]: connect from unknown[11.12.13.14]
Dec 1 11:16:21 mail postfix/smtpd[21197]: NOQUEUE: reject: RCPT from unknown[11.12.13.14]: 554 5.7.1: Relay access denied; from= to= proto=ESMTP helo=<banping.com>

自己搭建邮件系统还是比较折腾的，使用Amazon ses是比较便捷的做法。