Yes, send-mail-function is the only delayed-init defcustom that's autoloaded.

On Feb 23, 2010, at 1:04 PM, Glenn Morris wrote:

> send-mail-function's default value probably shouldn't be based on
> window-system anyway, now that X and tty frames can be mixed (though I
> don't know if this works on Windows/Nextstep). I think
> mailclient-send-it itself should check window-system and call
> sendmail-send-it if it is nil.

I don't think that this would be a user-friendly solution. 
It should be `initial-window-system', not `window-system', however.  I find that `browse-url' is not useful on remote emacs sessions via SSH.

You may recall that the default value is different on GNU/Linux because we expect users to configure their Emacs mail system explicitly (e.g., with a working sender address), or that the `browse-url' (with a mailto://) mechanism is less likely to produce the desired effects (haven't tested this).   Also, an older OS X version didn't correctly run `sendmail'.

The underlying reason for all of this is independent of the system - firewalls block outgoing mail traffic, and mail servers refuse to deliver mail from dial-up IPs.
I noticed all of this because a user complained that his bug reports didn't go through (but were silently swallowed).

And no, X/TTY mixing doesn't work on NS.