Automailer or
V-Mailer could be exactly what you want ... I haven't tried them myself, therefore I can't say anything 'boutthem.
Some full-fledged mail clients seem to be able to run from the command line as well, e.g.
Pegasus Mail or
Pine, but again I can't provide any details. (I use Pegasus personally, but never cared about such details. If everything else fails, I might try it out, though)