about mail

Depends on the account. If you are like me, you probably have a few email accounts set up in Mail. Google is keen on Archive and so is iCloud. In some cases you can go to accounts and select a different folder for the purpose listed.

You could set rules to automatically flag a message, all messages from work for example, then you can easily locate the important "Flagged" messages.

There are local, on the computer, and server delete mailboxes, if you have an IMAP type of account, then you probably have both. Some folders will let you delete them, others are required by the mail account and don't have an option to delete.