If you are getting below error message in SquirrelMail:
Error :-
ERROR: Could not complete request.
Reason Given: Unable to open this mailbox.
Do, the following things :-
login as a root
[root@server1 ~]# cd /home/UserName/mail/Domainname.com/emailIDUsername/
create new Directories as below
[root@server1 ~]# mkdir new; mkdir cur; mkdir tmp
After that change the ownership the directory to cPanel userver name
[root@server1 ~]# chown username.username*

That’s all you are done. 😀