Off topic: smtp HELO question

[Quartz]

This isn't directly related to FreeBSD, but I figure many people here 
probably run it as a mail server so someone might know the answer to 
this question.

By default, if you're behind a NAT, Thunderbird sends your local IP 
address as part of the 'hello' when connecting to a mail server, which 
then gets stamped into the header info for all to see as the email is 
sent down the chain.

I'm trying to debug some email issues, and I suspect that this initial 
header might be part of my problems. I can configure Thunderbird to send 
an arbitrary string instead of a NAT IP via the 
mail.smtpserver.smtp*.hello_argument variable, but I'm not 100% sure 
what I can legitimately put here without getting my emails marked as 
spam. Does this field have to match the reverse-lookup up of the 
world-routable external IP that you send the email through, or can it be 
any arbitrary string that matches a domain name pattern? Can anyone 
point me to a resource that explains this in depth?