unpair macmail from old mac from new mac

I migrated from my old mac (running Sierra) to a new mac running Monterey. I want to keep the old mac to give to one of my children to use but I do not want to keep the macmail accounts currently synced between the old and new macs, but don't want to lose the emails on the new mac since if I erase them on the old mac they will be erased from new mac - they are all imap accounts. Any suggestions on how to "unpair" the macmail on the devices while keeping the emails on the new mac? I use cox cable as my ISP.