It probably means that cron sent 21 bytes through sendmail, and sendmail
(which really is msmtp) returned an exit status of 0x0040 = 64 decimal =
EX_USAGE (command line usage error). I wonder which options vixie-cron
uses to call the sendmail binary. msmtp should understand all sendmail
options, but maybe I missed something.
... OK, I just checked the vixie-cron sources, and it seems that
vixie-cron uses the -F option of sendmail to set the real name of the
sender to "CronDaemon". msmtp does not yet understand this option.
I will add "support" for this option in the next version: msmtp will
silently ignore it, because it can do nothing meaningful with this
information.
Until then, you could change the vixie-cron sources to avoid the -F
option (you're a Gentoo user, right? :)
Remove the "-FCronDaemon" from line 44 in file config.h and recompile
the thing.