OK first you need to get the string. In this case
"--------- Original Message -------------"
Next you need to find the first occurence of the string using the following function:
strpos($fullmessage, $needle);
where $fullmessage is the email text and $needle is the original message string.
this function returns the offset as an integer.
Ok then we need strip out everything from $needle onwards like you wanted
So we use:
substr($fullmessage, $start, $offset);
where $fullmessage is the email text, $start should be 0 and the $offset we get from strpos(); but in this case we remove 1 from the $offset. So $offset should be $offset - 1. The reason for this is to remove the first character of "----------- Original Message -----------".
I hope this is what you where looking for