Can't deliver some mail...

Hi,
I've developed a problem sending mail. I'm sending it to my account on
my ISP, and it's barfing. Of course, I can still relay mail via my ISP,
so I'm stumped.

Well, I'm mainly stupmed at come it is sending from
"machine.localdomain" rather than from "machine.mydomain". If I could
work that out...

Anyway, here's a failure message, domain names and so on redacted:
--------------------------------------------------------------------

This is the mail system at host machine.localdomain.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<username@isp.domain>: host mx.isp.domain[a.b.c.d] said: 550 5.7.1
<user@machine.localdomain>... HELO machine.localdomain from RFC 2606
reserved domains blocked (in reply to MAIL FROM command)



Reporting-MTA: dns; machine.localdomain
X-Postfix-Queue-ID: ED92F6398C
X-Postfix-Sender: rfc822; user@machine.localdomain
Arrival-Date: Tue, 22 Apr 2008 15:28:49 +1000 (EST)

Final-Recipient: rfc822; username@isp.domain
Action: failed
Status: 5.7.1
Remote-MTA: dns; mx.isp.domain
Diagnostic-Code: smtp; 550 5.7.1 <user@machine.localdomain>... HELO
machine.localdomain from RFC 2606 reserved domains blocked