5.1.0 - Unknown address error 554-'5.7.1 Service unavailable; Client host [212.1

[#1]

I am getting the following error message when I email one particular address:
The following message to <****@******.org.uk> was undeliverable..
The reason for the problem:
5.1.0 - Unknown address error 554-'5.7.1 Service unavailable; Client host [212.159.7.100] blocked using ix.dnsbl.manitu.net; Spam sent to the mailhost mx.selfip.biz was detected by NiX Spam at Wed, 03 Nov 2010
04:57:03 +0100, see
http://www.dnsbl.manitu.net/lookup.php?value=212.159.7.100'
Can anyone help me with this?  I have tried emailing the same address from my hotmail account and it goes through fine. 

[#2]

The BL in "ix.dnsbl.manitu.net" means Blacklist. It means Plusnet's IP address appeared on a spam blacklist so the destination server rejected it. I clicked on the link provided and it is showing as 'not listed', which means they took it off or PN sorted it out. Try sending the email again.