[plug] postfix issue

[Jon Miller]

I'm getting a lot of the following in the /var/log/message file:
> Jan  1 15:40:15 mail postfix: Process did not exit cleanly, returned 255 with signal 0
> Jan  1 15:40:29 mail postfix: Process did not exit cleanly, returned 126 with signal 0

I'm told that I need to use the Postfix sendmail command instead because the postfix command should not be used for mail submissions.
Exactly what and where is this postfix sendmail command?

Thanks