If this message is a part of a fragmented message (i.e. has a
Content-Type of message/partial), an array with three elements
is returned.
The first element is an identifier string. This string should be used to
group this message with the other fragments of the message (which will
have the same id string).
The second element is the sequence number of this fragment. The first
part will have number 1, the next number 2 etc.
The third element of the array is either the total number of fragments
that the original message has been split into, or 0 of this information
is not available.
If this method is called in a message that is not a part of a fragmented
message, it will return 0.