I've added this host to the 'mailhost' setup (http://www.spamcop.net/mcgi?action=mhedit;authcode=blahblahblah) and I've received the confirmation email and clicked the link in the email. I get the above error every time I submit spam from that account.

My ISP is a relativly small-tme operation and their tech guy likes to dink with the code. I suspect he's screwed something up and it'll be like pulling shark teeth to get it changed.

On the screen explaining what the mailhost setup was all about, there was mention of a way to undo it and go back to the way I was using (which WORKED). I need to know HOW, please.

The procedure to delete a host? Messed Up My Mailhost Registration is one place the specifics are mentioned. I just used the search facility at the top of the page and the words delete mailhost. That post is a couple of years old but it still applies, by the look of it.