Transmits an e-mail message using the system's mail transport agent.
This function takes a message, a list of recipients, and an optional sender,
and transmits it using the system's MTA, sendmail.
If sendmail is on the PATH, it will be used; otherwise, a list of system
default locations will be searched.
A failure will be logged, since this function uses safeSystem
This function will first try sendmail. If it does not exist, an error is
logged under System.Cmd.Utils.pOpen3 and various default sendmail locations
are tried. If that still fails, an error is logged and an exception raised.