Insufficient space in home folder during Mail import on iMac

I hope to receive a method to solve this problem. I started a rebuilt of my mailboxes. On my screen I received a message telling me that Mail shows new featurers. I clicked continue and I send a screenshot of what happens during the import of messages. Please help me with a good method to have my messages back again. Morever the application Mail does not work anymore on my iMac. Frustrating

I got an advice to check the space availability. I believe there is enough space. Moreover I added no new messages

Thanks

R. KINGET

[Re-Titled by Moderator]