Tell me how to disable long-entered mail accounts so that iMessage cannot be used through them?
If you have connected several mailboxes to your Apple ID account, each of them can act as a separate account in the iMessage service.
To disable receiving and sending messages on a specific device , go to Settings – Messages – iMessage and disable unnecessary accounts.
You can disable accounts, you do not need to receive messages from them, and choose from the remaining ones the main one, through which the default correspondence will be kept.
In order to remove accounts from Apple ID altogether (for example, if mail has not been used for a long time or other people have access to it), go to the Apple ID account management service and log in.
In the contact section, click Edit .
Here you can delete all added mailboxes except the main one, which acts as an identifier.
If necessary, the remote box can always be tied back to the Apple ID.