I have setup the mail add-on and the mail account

I figured out how to attache the email to a lead or contact but I don't know 

how to attache it to an exsiting potential?

how to attache it to a new potential?

Is there any way the crm track the emails related to the potential and keep record about the correspondence automatically when I send/receive more emails related to this potential?

I usually corresponde with the same customer on multiple deals (potentials) will it be a problem