How do I fix a corrupt mailbox on my website's cPanel?

I'm hosting a reseller to help some friends with hosting I for whatever reason I started using a 2nd account with my websites domain for my personal business, when I went to retrieve an e-mail I got an error saying it was corrupt...I tried all of the options that the root admin had installed (ie: roundcube, horde, and squirrel mail) with no luck...what can I do to retrieve them?