Why official transcripts should be directly posted to the employer? Official transcripts are only those that arrive directly from the registrar's office. What is the concern about making sure transcripts are official? Fake or tampered transcripts can be sent by post too.

What are the security guarantees of a directly shipped transcript?
There are a number of security concerns:  


The postmark will match that of the university 
Many universities' official transcripts contain authentication features
that can't be easily replicated -- deterring fakes.
The receiver can contact the university to verify that the transcript
was indeed sent on the date listed on the transcript and that the details inside are correct.
The transcript may contain a letter from the registrar with a signature.
As in responses 2 &amp; 3 this can be used to make authentication possible while
deterring duplication.


The question I'm left with is why do you ask?