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On an IPv6-only machine, smtplib does not send an IPv6 address in the 
EHLO. It sends "EHLO [0.0.0.0]" instead. For example, on a v6-only 
network with stateless autoconfiguration enabled (a common 
configuration), smtplib will show this error.
Further, SMTP's __init__() method tries to determine the value of EHLO 
parameter before the socket is established. But it can't know what 
value to use before the socket is setup (e.g., should it use a 
hostname, a v4 address literal, or a v6 address literal).
The attached patch moves the self.local_hostname processing into the 
connect() method, and updates it to use IPv6-aware resolver interfaces.

I intended testing this but the patch file appears to have a dangling else at the bottom, or am I missing something?

You're right. My mistake when I made the patch. I've attached a new one without the trailing else

Ran test_smtplib.py for Python 2.7 and 3.1.2 on Windows Vista all tests passed.

The patch still needs unit tests.

I'm not sure how to best write a unit test for this. The patch should only be invoked if the client doesn't have a name-served hostname and has IPv6 enabled. Also, there are several IPv4-isms in the smtplib tests.

You might want to take a look at Lib/test/test_ftplib.py which contains IPv6 tests including the necessary logic to decide whether or not running them.