How to delete all Terminal mail

It you want to send an email to a given address put it as an argument to the mail command:

mail -s "Some subject" toaddress@example.com

see man mail for other options

Any text you type after issuing the command you can enter the text for your mail directly in the terminal. To end mail text entry, enter a new line with a single ..

Do note that this just queues the mail for sending as if your mac were a functioning mail server. Local delivery to local accounts is easy to set up, to set up a working mail server that connects to the internet requires some more advanced set up as well as a relationship with someone else that operates a mail relay. This isn't a client that works directly with mail services like gmail / yahoo / hotmail or iCloud (MobileMe).

In the GMAIL settings, it says show or not the All Mail folder in IMAP:

So, it is your choice to show the All Mail folder on your computer or not.

There is nothing to break by choosing which folders will show on your computer from mama google's web email.

The All Mail folder is a feature of the Gmail, not your email program. So if you do not want to see it locally then disable it. Also All mail is just a bucket where a duplicate from all other folders reside for easy search. It is not real folder, just a collection of emails from other folders.