Mail doesn't go

[Dion E Viglione]

Hi, I'm using fetchmail to get my email from my
ISP, and reading it with mutt.  I want to send
messages, but it says:

  Error sending message, child exited 65 (Data
  format error.).

and I receive an email that says:

  ...Domain of sender address does not exist

I understand that Mutt isn't like Pine, but I
thought sendmail can send stuff out...?  Is it
Mutt that I have to configure or sendmail?  Or
use yet another program to send the email out?

--Thanking you in advance,
Dion.