I am using the Postie plugin for a couple of WordPress sites. But in one site, it literally took my hours and lot of coffee to fix it. Note I was using the mail server as mail.mysitename.com (Read mysitename as my domain)
Our store uses the Script Opencart. It was showing blank page while tried to access Admin page.
I have pasted the following lines in Index.php of Admin directory and checked for errors.
After that while reloading the Admin page, I found the error was in the directory path. After renaming the correct directory name in Config.php inside Admin folder, it started working fine..
Finally the blank page problem solved!