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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.