If you are getting below error message when verify the e-mails with SquirrelMail:
Error :-
The server was not able to find the document (./3rdparty/squirrelmail/index.php) you requested.
Please check the url and try again. You might also want to report this
error to your web hosting provider.
webmaild/11.25 Server at *

Do the folling things :
1) login as a root and create a backup of “squirrelmail” in case if there is another problem arise.
[root@server1 ~]# cd /usr/local/cpanel/base/3rdparty/squirrelmail
[root@server1 ~]# cp  squirrelmail  squirrelmail.bak

2) After that give the below Command
[root@server1 ~]# /usr/local/cpanel/bin/update-squirrelmail –force

That’s it issue resolved 😀