1.sudo apt-get install squirrelmail
2.sudo cp /etc/squirrelmail/apache.conf /etc/apache2/sites-available/squirrelmail.conf
3.修改/etc/apache2/sites-available/squirrelmail.conf文件，去掉部分注释，并将ip和域名填写上去：
# users will prefer a simple URL like http://webmail.example.com
<VirtualHost 192.168.0.26:80>
DocumentRoot /usr/share/squirrelmail
ServerName webmail.test.com
</VirtualHost>
4.sudo ln -s /etc/apache2/sites-available/squirrelmail.conf /etc/apache2/sites-enabled/
5.service apache2 reload
6.用ip登录服务器，可以看见squirrelmail的登录页面。