Yes, this has the same root cause as the failure in Issue20605 since SMTPServer in smtpd.py uses getaddrinfo.  I'm now able to reliably reproduce the failure.  The system getaddrinfo failure is seen when the OS X 10.6 system's network configuration is *not* using the local mdns for its primary domain service (which can be checked with "scutil --dns"); it fails when using an external dns as its primary dns service.  At least that's one failure scenario.  In any case, this seems to have been fixed in later versions of OS X, the problem appears to be unique to getaddrinfo (gethostbyname works OK), and it's only this one test.  I'm tempted to just close this as "won't fix"; on the other hand, it's easy enough to change this test to use '127.0.0.0' instead of 'localhost'; there are precedents for doing that for other reasons (Issue18792, for example).  Here's a patch that does so and thus avoids the potential problem on 10.6.  I'll apply it if there are no objections.