Clara are idiots.
My business used to be with them. Then, one day the connection went. I made some phone calls and Clara said they disconnected it, due to an allegation of copyright infringement with P2P, referring to P2P traffic a month previously.
Setting aside whether the allegation was true
I asked him who it came from. He said he can't say due to the data protection act.
I asked what evidence they ask for; reply was "any complaint".
I asked how was the complaint delivered; reply was "email".
I asked why did they not contact me first. Reply was "our terms and conditions don't require us to" (actually that is true for just about every ISP).
I asked "does this mean I could send you a forged email, pretending to be from say Microsoft, alleging that one of your large business customers [of which Clara has a fair number] has been found to have some P2P traffic on which a Microsoft product was downloaded/shared, and you would summarily, without notice or any communication, disconnect their internet service?" Reply was "Yes".
Decent ISPs, e.g. Zen, first send an email back to the complainant (which is nearly always an automatically generated email BTW) asking them for evidence of the alleged copyright infringement. Nearly always, this evidence is never delivered (I have this from the horse's mouth). Many ISPs, including BT Internet I am told, get so many of these emails they just ignore them.
ISPs that cut off what they call abuse invariably do it to keep down traffic. Not a lot of people know this but the standard flat-rate products are not backed by a flat rate deal from BT. In general terms, the ISP has to pay for the bandwidth in specific chunks, and has to pay for the # of concurrent connections, so him selling flat rate deals is a business bet on not picking up too many users who use too much bandwidth.
Back to the original topic, it is amazingly common for a whole big ISP to be blacklisted. What happens is that the ISP gets inundated with phone calls from irate customers whose emails are not reaching their destinations, and somebody with hopefully more than 2 braincells realises they got blacklisted, so they contact Spamcop (and others) and sort it out, usually within a day or two. But a small ISP, or one in a 3rd world country (e.g. anywhere in Latin America, the Far East, etc) could be blacklisted for weeks or longer. A lot of smaller American ISPs are blacklisted almost continuously.
But the biggest reason for "lost emails" is the destination's spam filtering. In fact most internet users don't even know their email provider / corporate email system is doing spam filtering. This can take many forms. For example one contact I once had, in a big London bank, would never get an email whose text (this was a plain text email without any attachments) contained the string ".exe".
Can't beat fax
