You should only really worry if the person says they're from Nigeria, and you receive an email like this:
MELLOW GREETINSG
I AM THE 5TH IN LINE TO THE THRONES OF THE KING OF NIGERIA. I HAVE BEEN TRUSTED WITH BUYING A GIFT FOR HIS 7TH SON THE PRINCE MUHAHAHAHAHBABABA UPON HIS HOMECOMING FROM HIS TOUR IN THE U.S. YOUR CRAPPY MOBILE 2ND HAND MOBILE OHONE WOULD BE AN IDEAL GIFT FOR HIS ROYALNESS AND I NEED VERY MUCH. I WILL SEND YOU A CHEQUE FOR 2000 £ MORE THAN IT'S WORTH, AND YOU SEND ME BACK THE DIFFERWENCE OK.
MAY GOD BLESS YOU.