The email generated by up2date has bogus "From" addresses -- either
"up2date@localhost" or "root@localhost" (client.py, lines 88 and 105).
Paranoid MTAs will reject this, even as a local message: after all,
"root@localhost" is not a valid email address.  Probably a slightly more
general solution would be to put the FQDN ("hostname -f") as the
domain of the address -- eg. replace 'localhost' with "os.uname()[1]" -- of
course, that assumes that the machine is configured so uname() returns the
FQDN, but even so it's better than hard-coding "localhost" (IMHO).