sendmail X is a message transfer system that has been designed with
these main topics in minds:

- security
- reliability
- efficiency
- configurability
- extendibility

sendmail X consists of five main modules of which only one runs
as root:
 
- mcp: the main control program is similar to inetd(8): it starts
  all other sendmail X modules and watches over their execution.
  mcp runs as root in order to bind to port 25 and to change the
  uid of the processes it starts.
- smtps: the SMTP server receives e-mails.
- smtpc: the SMTP client sends e-mails.
- smar: the address resolver provides lookups in various maps including
  DNS for mail routing.
- qmgr: the queue manager controls the flow of e-mails through the SMTP
  servers and clients.

WWW: http://www.sendmail.org/