I don't know a single ISP who archives personal emails. While ISPs will back up whatever emails are left on their servers every day, they don't systematically archive every email that goes thru.
Corporate, yes, ISPs no.
Why do I know this ? I run services or consult to a bunch of companies in the ISP business.
http://gmail.google.com/mail/help/more.html#data
"Some news stories have suggested that Google intends to keep copies of users' email messages even after they've deleted them, or closed their accounts. This is simply not true. Google keeps multiple backup copies of users' emails so that we can recover messages and restore accounts in case of errors or system failure. Even if a message has been deleted or an account is no longer active, messages may remain on our backup systems for some period of time. This is standard practice in the email industry, which Gmail and other major webmail services follow in order to provide a reliable service for users. We will make reasonable efforts to remove deleted information from our systems as quickly as is practical."
to emphasize:
"Even if a message has been deleted or an account is no longer active, messages may remain on our backup systems for some period of time. This is standard practice in the email industry"
so, you have 100% faith in their "reasonable efforts" to remove deleted information?