use of email in mail function

Hi guys,

Sometimes i do recieve some email from somebody that doesn't belong to our company...

for instance my company email end with @mycom.com

i did recieve some email with dsadas@mycom.com...

is it they use mail function and set the from header with that email address ?? is there anyway i can prevent unauthorize people from doing it ??

my friend said there is no way he is can get any reply if the account is not there but there is also no way we can prevent such people from doing it.. is that so ??

please advise.