Mail on my M1 iMac

I have two Mailboxes, Gmail and iCloud. No problems with Mail on my iPhone 8 or my M1 MacBook. But on my 2 year old M1 iMac there is a strange thing occurring, concerning the iCloud-postbox, not the Gmail-postbox. In the Sent-map a lot of junk messages pop up, messages that I definitely have not sent. This problem does not occur on my iPhone or M1 MacBook. In the Sent messages-map this does not occur. Mostly, sending messages works just fine, both using Gmail and iCloud. I try to delete all these mysterious messages in the Sent-map, but new messages steadily occur. What is wrong??

Questions:

1. What is the difference of Sent-map and Sent messages-map in the iCloud-postbox?
2. How can I prevent these junk messages to pop up in the Sent-map?
3. Is it possible to delete the Sent-map and reinstall it?