I wouldn't use ereg_replace when str_replace will work here.
$var = str_replace("\r\n", "<br>", $var);
However, if you aren't sending HTML mail, you don't want to replace the newline chars with a break tag anyway.
I use something like this and it works fine.
mail("email@domain.com", "Subject", "Some text \n\n more text","from:whoever@domain.com");