E-mails not going to proper folder

I have a folder that is setup in Public folders call Internship. This folder should be receiving e-mails external, but we have not been getting them for a will, it has work in the past, for the last month or so we have not got any e-mails to that folder. My users are telling me that people have been sending stuff to that e-mail address, but some how it will not show up. So what is the next step I got take to fix this issue, hope you can help me out?