Emails being returned?

[#1]

3 times today (out of nowhere; never had this before), I've had 3 emails returned. Two had the following message:
SMTP error from remote mail server after initial connection:
    host mx.vip.mistral.kcom.com [212.50.160.75]:
    554-mercury.karoo.kcom.com
    554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means.
The other (sent on a different email address, but still routed thro' Pnet I think):
SMTP error from remote mail server after RCPT TO:<email address>: host organicadvice.org.uk [205.214.86.24]: 550-"JunkMail rejected - nairn.just4dns.com [78.129.134.15] is in an RBL, see 550 Blocked - see http://www.spamcop.net/bl.shtml?78.129.134.15 / Return-path: <images@sarahjameson.co.uk>Received: from newinvention.plus.com ([80.229.182.234] helo=mrstdl6hiwff8p) by nairn.just4dns.com with esmtpa (Exim 4.69) (envelope-from <images@sarahjameson.co.uk>)
I can supply full header if anyone can make sense of it!
I don't use my Plusnet email address any more - but I understand that all my emails get routed through Plusnet.
Does this signify that my PC has a virus or is something else going on? And is the problem with me, with the outgoing server, Plusnet or incoming server?
Thanks for any guidance!

[#2]

OK, it would help to know where you're sending from, where you're sending to and what outgoing server address or Webmail client you're using?

Quote

SMTP error from remote mail server after initial connection:     host mx.vip.mistral.kcom.com [212.50.160.75]:     554-mercury.karoo.kcom.com     554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means.

That could be a problem with one of our outbound mail servers however I can't really say for sure without seeing the full headers of the returned message.

Quote

SMTP error from remote mail server after RCPT TO:<email address>: host organicadvice.org.uk [205.214.86.24]: 550-"JunkMail rejected - nairn.just4dns.com [78.129.134.15] is in an RBL, see 550 Blocked - see http://www.spamcop.net/bl.shtml?78.129.134.15 / Return-path: <images@sarahjameson.co.uk>Received: from newinvention.plus.com ([80.229.182.234] helo=mrstdl6hiwff8p) by nairn.just4dns.com with esmtpa (Exim 4.69) (envelope-from <images@sarahjameson.co.uk>)

That looks like the server 'organicadvice.org.uk [205.214.86.24]' rejected the email because it was being sent from the server 'nairn.just4dns.com [78.129.134.15]' which has been blacklisted by Spamcop.

[#3]

Hi Bob - yes you were right. I managed to get in touch with the server and they confirmed that someone had used their email account to send loads of spam today so the whole server was blacklisted by Spamcop. Hence none of my emails were getting through. I think the matter is resolved now. But thanks for your reply. 

[#4]

newinvention - please edit your posts to remove or blank out the email addresses. This forum is publicly viewable, so any email addresses can be harvested by spambots.