postfix/mail: server blacklisted -- what have I done wrong?

[Matt Price]

Hi folks,

On a very small server running postfix and apache under breezy, I seem
to have gotten into some kind of trouble.  A mail sent this morning from
the system was bounced back from gmx.de with the following message:

host mx0.gmx.net[213.165.64.100] said: 550-5.7.1 {mx016}
    The IP address of the server you are using to connect to GMX is
listed in
    550-5.7.1 the Composite Blocking List (CBL). 550-5.7.1  550-5.7.1For
    additional information, please visit 550-5.7.1
    http://cbl.abuseat.org/lookup.cgi?ip=128.100.34.8  and 550 5.7.1 (
    http://www.gmx.net/serverrules ) (in reply to RCPT TO command)

-----------------
so, I followed the link, which ultimately led me to this page about HELO
misconfiguration:
http://cbl.abuseat.org/lh.html

Still wish I knew what "HELO" was about, but at least it helps.
Turns out that I I had been having some trouble in the last few weeks
with rejected mail complaining about "localhost.localdomain".  Finally
fixed it a few days ago by editing /etc/postfix/main.cf 
----
# myhostname = localhost.localdomain
myhostname = a.legitimate.domain.name
----

so a couple of questions follow for me:

1 - is this the likely cause of the problem?  THat is, have I already
fixed a problem that has only just now arisen?
2 - If localhost.localdomain is ALWAYS a broken value for myaddress, how
does it come about that this is even a configuration option?  SHould I
for instance file a bug?

Thanks for your help,

Matt