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创建自己的CA文件，如下所示：

$  /etc/ssl/misc/CA.pl -newca
CA certificate filename (or enter to create)

Making CA certificate ...

Enter PEM pass phrase:CAPASSWORD
Verifying - Enter PEM pass phrase:CAPASSWORD

Country Name (2 letter code) [AU]:CN
State or Province Name (full name) [Some-State]:Shanghai
Locality Name (eg, city) []:Shanghai
Organization Name (eg, company) [Internet Widgits Pty Ltd]:LiaoJL.com
Organizational Unit Name (eg, section) []:
Common Name (eg, YOUR name) []:LiaoJL
Email Address []:liaojl@liaojl.com
... ...

默认密钥的有效期为一年，为了避免麻烦，将有效期更新为三年

$ openssl x509 -in demoCA/cacert.pem -days 1024 -out cacert.pem -signkey demoCA/private/cakey.pem
$ cp cacert.pem demoCA

建议编辑/etc/ssl/openssl.cnf文件，修改默认有效期为十年：

default_days=3650

接下来，通过CA生成服务器证书文件，"-nodes"参数用于避免服务器每次重启时要求输入密码的问题。

openssl req -new -nodes \
      -subj '/CN=LiaoJL/O=LiaoJL.com/C=CN/ST=Shanghai/L=Shanghai' \
       -keyout FOO-key.pem -out FOO-req.pem -days 3650

注意："/0=LiaoJL.com"必须和证书文件完全吻合。

为刚才生成的证书添加签名：

openssl ca -out FOO-cert.pem -infiles FOO-req.pem

Postfix需要的证书和密钥文件都准备好了，将它们复制到Postfix配置文件目录中

$ cp demoCA/cacert.pem FOO-key.pem FOO-cert.pem /etc/postfix
$ chmod 644 /etc/postfix/FOO-cert.pem /etc/postfix/cacert.pem
$ chmod 400 /etc/postfix/FOO-key.pem

我的配置文件如下：

## TLS Settings
#
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_tls_cert_file = /etc/postfix/FOO-cert.pem
smtp_tls_key_file = /etc/postfix/FOO-key.pem
smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_tls_session_cache
smtp_use_tls = yes
smtpd_tls_CAfile = /etc/postfix/cacert.pem
smtpd_tls_cert_file = /etc/postfix/FOO-cert.pem
smtpd_tls_key_file = /etc/postfix/FOO-key.pem
smtpd_tls_received_header = yes
smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_tls_session_cache
smtpd_use_tls = yes
tls_random_source = dev:/dev/urandom

##  SASL Settings
# This is going into THIS server
smtpd_sasl_auth_enable = no
# We need this
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/saslpass
smtpd_sasl_local_domain = $myhostname
smtp_sasl_security_options = noanonymous
#smtp_sasl_security_options =
smtp_sasl_tls_security_options = noanonymous
smtpd_sasl_application_name = smtpd

## Gmail Relay
relayhost = [smtp.gmail.com]

# Disable DNS Lookups
disable_dns_lookups = yes

#
transport_maps = hash:/etc/postfix/transport

更多详细内容，可以参考手册：http://www.postfix.com/TLS_README.html

依照上面的配置，Postfix从Berkeley DB或者说Hash文件中读取SASL验证时的用户名和密码信息，在/etc/postfix/saslpass文件中添加：

smtp.gmail.com liaojl@liaojl.com:secret

然后，执行postmap命令，生成saslpass.db文件。

$ postmap saslpass

新建/etc/postfix/transport文件，指定所有邮件通过Gmail即smtp.gmail.com转发。

liaojl.com               smtp:[smtp.gmail.com]

重启Postfix使配置文件生效，从主机上发送邮件可以通过Gmail转发，如下所示：

Oct 23 23:02:39 liaojl postfix/qmgr[7838]: 1BAE8A506: from=<root@liaojl.com>, size=274, nrcpt=1 (queue active)
Oct 23 23:02:42 liaojl postfix/qmgr[7838]: 1BAE8A506: removed
Oct 23 23:02:42 liaojl postfix/smtp[7897]: 1BAE8A506: to=<daniel.liao@live.cn>, relay=smtp.gmail.com[209.85.143.109]:25, delay=3.1, delays=0.05/0.04/1.5/1.6, dsn=2.0.0, status=sent (250 2.0.0 OK 1224774161 b4sm470754tic.2)