确保使用了 FEATURE(`mailertable') , redhat默认是使用了的。
1. 
cat >> /etc/mail/mailertable <<EOF
126.com      smtp:mx.chengxin.126.com
163.com      smtp:mx.chengxin.163.com
yeah.net     smtp:mx.chengxin.yeah.net

EOF

2. 
makemap hash /etc/mail/mailertable < /etc/mail/mailertable


3. 
/etc/init.d/sendmail restart

4. 测试向yeah.net用户发信，并检查/var/log/maillog日志：
Dec 20 14:47:43 163mx4 sendmail[17431]: qBK6l9ZN017431: from=<fff@fff.com>, size=80, class=0, nrcpts=1, msgid=<201212200647.qBK6l9ZN017431@163mx4.163.com>, proto=ESMTP, daemon=MTA, relay=163mx4 [127.0.0.1]
Dec 20 14:47:44 163mx4 sendmail[17453]: qBK6l9ZN017431: to=<wenfeng@yeah.net>, delay=00:00:21, xdelay=00:00:01, mailer=smtp, pri=120080, relay=mopen.yeah.net. [123.58.177.11], dsn=4.0.0, stat=Deferred: 421 Too many connections

relay=mopen.yeah.net. [123.58.177.11],
日志行中的relay=mopen.yeah.net 表示邮件从专门通道发送了