To delete the mailbox, you need to open the "Mail" section, click the "More" button on the top right side and select the box that you want to delete. Click "Disable Mailbox" - select "Delete Emails".
In this case, the note notes will remain in the card, but will be unreadable (they will only be available on your server).
If you want to temporarily disable or re-enter the password, you need to "Disconnect the mailbox" without deleting the messages. Then the letters will remain in the system.