A lot would depend on whether they ever backed up their mail server, and what kind of server it was. If they were following best practices (and with this bunch, that’s a BIG “if”), then yes, there would be backups of the email system with those emails. However, sometimes “sent” mail is not backed up, so unless you have backups available from all of the mail environments at each location (recall that not all of these “scientists” are in England, many are in other countries), you may not be able to recover all of a particular chain of emails.
Also, it is possible that they have backups, but none are valid (if they never did a restore to ensure the backup worked), or that they keep a very small window of backups (only 2 weeks’ worth, for example) for emergency restores, or even that they have deliberately erased them along with all the other original data they used.
Since Jones did a mass email delete in October 2008 there might be a good chance that the CRU has a backup of all the contents of their computers for the end of the third quarter 2008 or even a back up for the year end 2007.
Maybe another CRU whistleblower will sound off by giving the standard operating procedures used by the CRU for backing up their data.
Also, the fact that Phil Jones did a mass delete of emails in October 2008 could go a long ways in explaining why there were only slightly more than 1,000 emails covering a period of more than 10 years.