Looking back over the logs (via bbreport) I don't see any failures of test_smtplib that match the report in this issue.  There were three in the last while, two of which look like network issues on the same machine (and thus probably not a test problem per se) and one of which (solaris) had multiple errors but succeeded when rerun immediately.  

While there maybe a remaining flakiness issue with test_smtplib, it doesn't seem to be the error described here, and I don't think it rates critical.