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Courier Mail Server, also known as Courier MTA, is a free/open-source message transport agent (MTA) which provides POP3 and IMAP services (among others).
It is available in standard repositories for the Ubuntu family of Linux distributions (beginning no later than version 8.04).
To get couter-webadmin (the web-based configuration app) to manage hosted domains, it is necessary to create the folder hosteddomains/webadmin inside the existing folder /etc/courier, and give the application write permissions (777 works, but lesser permissions may work as well if the group and/or user are set properly).