Raised issue below as wish list item, but since it just happened on updating to 2.6 (150 emails downloaded for the third time!) as well as on first installation I think the underlying issue must be a bug not a design feature.
Many thanks & best wishes
Mouse
Many people hold the last n days of emails on their email server, even after they have downloaded them. (There is an Outlook option to do this).
If you are one of these people and you install (or update) CAS, then CAS downloads duplicates of all the emails you have already received during the last n days. It also send authentication messages to everyone not on your trusted senders list, which, at this early stage is unlikely to be complete. This can seem to (but not actually) completely lock up CAS, slow down you computer substantially, and (because of another issue raised above) constantly seize focus, making people think CAS is buggy - not a good introduction.
SO CAS needs to offer the installation option to inhibit or delay this behavoir. Delaying could be useful, giving people the option to add important names to their whitelist, and then build it further by running CAS against the last n days of emails
Best wishes
Mouse