At home I've a PCs with an external modem, also 've an email address (address@isp.com) of my ISP.

The PC has freeBSD with three accounts, I need to fetch all mails from address@isp.com to distribute them to each account.

My doubt is how to know what mails are from one account or for other one if all email arrives to the same address (address@isp.com).

Is it possible, how to do that?
Please don't limit your explanations, I mean, in technical words.
Thanks in advance...