Keeping saved emails when stopping using an email account
If I delete my yahoo email on my mail app on my iMac and stop using the account will the yahoo emails stored in iCloud folders still be accessible? It is important to me to keep my old emails
If the mail messages are stored in folders created in the hierarchy under On My Mac, the messages are stored locally on the Mac, and are not affected by whatever might happen with the mail server.
If the mail messages are relocated to folders on another mail server such as iCloud, they will also be preserved.
It is also possible to export mail messages into a file using mbox format, and that will get picked up by Time Machine or other backups.
Import or export mailboxes in Mail on Mac - Apple Support