Ask Al: Is my personal domain affected by DMARC?

All this talk about Yahoo's recent DMARC policy change got a friend to ask me about her domain name and whether or not this change has any impact on her.

Ellen asked me, "Does this mean anyone with a personal domain sending through an ISP who implements DMARC with a p=reject policy is going to have problems if they try to send mail to any recipient who checks DMARC?"