This is happening with Darcs 2.12.5 and current Darcs HEAD.
When no MTA (Mail Transport Agent) is installed, `darcs send` fails
right after computing the list of patches to be sent, even if it is used
without the --mail flag (ie, when you do not need an MTA at all):
$ darcs send
Creating patch to "http://darcs.net"...
Patch bundle will be sent to: firstname.lastname@example.org
No working sendmail instance on your machine!