How to stop all downloads before certain date

There are 80,000 messages to me stored on comcast.net. I cannot afford to erase or loose any of them. I want them to stay there forever. But I want eMclient to download to my computer eMclient area copies of all new messages that appear on comcast.net from this point onward. But no older messages.

How do I tell eMclient to do that?
Thank you.

In other words I want my eMclient Inbox to have only a few messages, just the new ones. And the online client (comcast.net) to keep all 80,000 old messages there.

It does not seem to be possible but maybe this article from the knowledge base helps:
https://support.emclient.com/index.php?/Knowledgebase/Article/View/170/5/can-i-limit-email-synchronization-to-only-a-selected-time-period