Specify "mail from:" in postfix + relayhost

[Mathieu Malaterre <mathieu.malaterre@gmail.com>]

Hi there,

  I've just finish setting up a postfix + relayhost conf thanks to :

http://wiki.debian.org/Postfix#Installing_and_Configuring_Postfix_on_Debian

  This works great. However using the `mail` command line interface I
can only send email from my main gmail adress. I did a small test
using `telnet localhost 25`, I can then specify any of my
pre-configured gmail adress (eg @free.fr).
 However using telnet is pretty tedious. Does anyone knows what is the
command line option to specify the 'mail from:' information ?

Thanks much,
-- 
Mathieu