Sometime Today, SD cobbled together some glyphs to say:
On my FC1 I'm able to receive & send mails using thunderbird but sending mail by /bin/mail does not work.
Configure sendmail or whatever is your local MTA to use a smart host. This is the same as what you've set in thunderbird as your smtp server.
/bin/mail uses the local sendmail program to send mail so that needs to know how to get to the internet.
Additionally, if your sendmail runs as a smtp daemon, you can tell thunderbird to use localhost:25 as your smtp server. This will allow you to send mails even when offline. Sendmail will queue them and send them when you connect to the internet.
Also, please don't use insanely large signatures. Internet netiquette dictates that signatures be no longer than 4 lines of no more than 80 characters each (so approx max 320 characters in a sig).