Undelivered mail to certain recipients.MAILER-DAEMON@orange.fr message



Version: 2004
Operating System: Mac OS X 10.4 (Tiger)
Email Client: pop

When I send messages to certain addresses I get back this message:

This is the mail system at host mwinf2710.orange.fr.

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

<...>: host mail.nhs.uk[212.137.44.179] said: 451
<....>... Requested mail action not taken: mailbox
unavailable (in reply to RCPT TO command)
&#65532;
Reporting-MTA: dns; orange.fr
X-SMTP-Server-Queue-ID: DBC911C0AA62
X-SMTP-Server-Sender: rfc822; nia.harland@xxxxxxxxx
Arrival-Date: Fri, 19 Sep 2008 08:12:17 +0200 (CEST)

Final-Recipient: rfc822;
Action: failed
Status: 4.0.0
Remote-MTA: dns; mail.nhs.uk
Diagnostic-Code: smtp; 451 <.....>... Requested mail action
not taken: mailbox unavailable

and another:

Un message a ete emis depuis votre compte de messagerie.

Certains destinataires n'ont pas recu votre message.
Vous trouverez en piece jointe le message qui n'a pu etre delivre.
Nous vous invitons a verifier l'adresse des destinataires et a essayer
de renvoyer le message.

This is the SMTP Server program at host orange.fr.

I'm sorry to have to inform you that your message could not be
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 SMTP Server program

<.....>: host mx2.uk.tiscali.com[212.74.100.148] said: 550 mail
not accepted from blacklisted IP address [80.12.242.145] (in reply to MAIL
FROM command)
&#65532;
Reporting-MTA: dns; orange.fr
X-SMTP-Server-Queue-ID: B24C3700008D
X-SMTP-Server-Sender: rfc822; my address
Arrival-Date: Sun, 7 Sep 2008 22:54:49 +0200 (CEST)

Final-Recipient: rfc822; ....
Action: failed
Status: 5.0.0
Diagnostic-Code: X-SMTP-Server; host mx2.uk.tiscali.com[212.74.100.148] said:
550 mail not accepted from blacklisted IP address [80.12.242.145] (in reply
to MAIL FROM command)

They are to addresses I have previously successfully sent to. They have not knowingly listed me as junk mail.
I am not very computer literate, so any help would be appreciated- given in the simplest step by step fashion. I don't even know how to contact my post master.
Thanks
.