Other mail clients, e.g. Pine, support sending mail by invoking sendmail or other MTA rather that by connecting them over TCP/IP. It may be needed if sendmail or other MTA is not running as daemon or it refuses to relay mail or access to the port 25 outside the LAN is blocked by firewall. Users may want to run sendmail over ssh or Kerberos rsh on machines where they have accounts rather than to feed their mail to broken sendmails without proper DNS records within the firewall.

it's an interesting idea. instead of going through the smtp protocol, we'd just exec a command and pass it the file we write to /tmp. if we ever did this, any changes to the ui would have to be UNIX specific, just like the eventual movemail UI will be. marking enhancement. pavel, I don't expect we'd get to this feature any time soon.

FWIW, I've used this to create and send test messages. You just create the entire message, including headers, and do something like cat message.txt |sendmail -t -U or msg-prog |sendmail -t -U or even a perl script. It's just piping.