There is really no way you can guarantee it came from Stateside. Hackers can take control of a computer in any country and send email from it, so it look like it came from that country.
yeah, dave is right. my cousin lives in SA and is well into IT. when my father was having trouble with his new laptop my cousin took over and sorted it out from SA
There was also a lad in my class in school that took to computers very well and found it fun to access random computers. unfortunately he got caught and is currently doing his 3rd 6 month stretch for it as far a I know