Problems with sending emails

Hello I am having problems sending emails to one particular address. I used to be able to send them OK, but recently for some reason I don't understand and neither do they, my e-mails bounce back with the following error message: 5.1.0 - Unknown address error 553-'5.3.0 Spam blocked see: http://spamcop.net/bl.shtml?62.24.202.76' Reporting-MTA: dns; out.ipsmtp4nec.opaltelecom.net Final-Recipient: ( email address blanked for privacy by me!) Action: failed Status: 5.0.0 (permanent failure) Remote-MTA: dns; [86.17.6.22] Diagnostic-Code: smtp; 5.1.0 - Unknown address error 553-'5.3.0 Spam blocked see: http://spamcop.net/bl.shtml?62.24.202.76' (delivery attempts: 0) I have no idea what this means. Can any one explain? Is this a fault on my computer or their - I think they use apple iMAc Many thanks in advance for any assistance Rarmy