Notes For RedHat and CentOS Users Exim4U was originally developed under the CentOS version of Linux. Therefore, Exim4U should work pretty much "right out of the box" on RedHat and CentOS systems. The instructions included with Exim4U were written specifically for RedHat/CentOS 5 which is no longer current. So, there may be some minor descrepancies in newer more current versions of RedHat/CentOS.