"From" aliases using mail and sendmail

[Malcolm Kay]

May user login is 'malcolm' whereas my ISP allocates 
me an e-Mail address 'malcolm.kay at internode.on.net'.

I can get internode.on.net into the domain of the 'from'
address using the masquerade feature in sendmail.

But how can I translate the user part:
  malcolm -> malcolm.kay
when using mail and sendmail?

I really need something like sendmail aliases but 
operating on outgoing 'from' addresses rather than 
incoming 'to' addresses. 

Malcolm