To use your normal editor within the mail program:
Enter the mail command in the usual way.
Enter the subject of the mail message.
Call up the editor by entering the command ~v at the start of line.
A file is automatically created in the directory /tmp to hold the contents of your message until it is sent.
Use your editor in the usual way to compose the mail message.
Exit the editor, saving the file in the usual way.
Send the message by entering a full stop at the start of a new line and pressing the <Return> key.