CITmail is a Hosted email solution based on open source code. It synchronises email, calendar and contacts between a hosted web server and the end users email client (Outlook or Thunderbird). The system is used by companies who do not wish to host their own mail server and instead prefer it to be managed and backed up remotely.
CITmail has the below functionality:
Implementation
For a technical overview of how email hosting services are engineered you can read about email hubs.
References
CITmail Wikipedia(Text) CC BY-SA