> Traditional Unix terminal- or command-line-based MUAs inject new messages
> into the outgoing mail queue by piping them to /usr/sbin/sendmail.
>
> Fancy GUI MUAs like Thunderbird are often written to work on either
> Windows or Unix, so they don't always offer the ability to inject through
> /usr/sbin/sendmail (because obviously Windows doesn't have that).
While I think this explains the history behind it, nowadays most MUAs
need to speak SMTP. This is not just because it has become commonplace
for an MTA to be missing even on Unix machines, but because
spam-checking basically enforces that *machines* aren't trusted to send
mail any more, only users (as an approximation of "human being") are, so
unless you're willing to store your personal password for use by the
system's MTA, the system's MTA will usually not be able to send your
mail to your recipients :-(
Stefan
