I spent a little time researching this and asked a systems person as well. The systems suggested the same file that you're reviewing -webmaillogout.cgi, but we have no specifics. I also found some discussion of the same issue on cPanel's forums (the people who provide the Webmail interface), but again, I did not see any specific solution. You may need to contact cPanel and see if there's any documentation available that lists the file and if its possible. I do apologize for not getting a clearer answer, but here's the link for the forum posts that I saw on the subject: cPanel Forum posts on webmail issue
. These are older posts, so there may be some newer information available. If I do run across anything, I will re-post to this comment and update it.