mail has two screens of online help that show you the available commands.
To get help when you are typing a message, go to a new line and type ~? (tilde-question mark). This is called a ``mail compose escape;'' you use these escapes to give a command to the mail program while you are typing a message. The help screen shown lists all the mail compose escapes.
To get help when you are reading your mail, type ? at the mail prompt. The help screen you see shows the commands available when you are reading your mail.