Instructions
Walk up to Warren when his attention is distracted and he is looking out the window. You can do this either at the end of the third or the sixth memory sequence. Press any button when prompted to steal his pen.
Play through the next memory as Altair and return to the Animus. Enter your room. This will make Lucy and Warren leave their computers unattended. Leave your room and go back into the central area with the Animus.
Walk up to Warren's computer and press any button when prompted to use it. You'll be able to read his outgoing and incoming emails.