Please do not recommend squirrelmail. It is not really being maintained.

https://www.openwall.com/lists/oss-security/2018/07/26/2

https://www.openwall.com/lists/oss-security/2018/03/17/2

https://squirrelmail.org/index.php

https://blog.cpanel.com/the-death-of-squirrelmail/