Correct value for EHLO when submitting mail

Hi,
when a MUA submits e-mail over SMTP, what value shall be used in the 
HELO/EHLO greetings? Shall I just stick with "localhost", or trust the 
OS-reported hostname, if any, or shall I use the IP address of my local 
endpoint?

I see problems with all of these, and apparently Thunderbird went through a 
nice discussion on this matter [1]. I didn't find any answer about that in 
RFC 6409, unfortunately, and RFC 5321's discussion [2] about this topic 
leaves me without a definitive answer, either -- the MUA will have zero 
idea about the "primary host name" and its relation to what OS returns, so 
I'm inclinded to interpret this as a suggestion to use address literals, 
but then I've heard about spam filters penalizing such messages [1]...

What is the current best practice?

With kind regards,
Jan

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=279525
[2] http://tools.ietf.org/html/rfc5321#section-4.1.4

-- 
Trojitá, a fast Qt IMAP e-mail client -- http://trojita.flaska.net/
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