The MAIL command tells the mail server to start a new conversation. It's also used to let the mail server know where to send a mail message to report errors. The syntax looks like this:
MAIL FROM:<reverse-path>
If the mail server accepts the command, it will reply with a code of 250. Otherwise, the reply code will be greater than 400.
In the example shown previously
>MAIL From:<(dave@cs.cf.ac.uk)> <250 <(dave@cs.cf.ac.uk)>... Sender ok
The reverse-path is different from the name given as the sender following the DATA command.
You can use this technique to give a mailing list or yourself an alias. For example, if you are maintaining a mailing list to your colleaguse, you might want the name that appears in the reader's mailer to be 'MyNickname' instead of your own name.