mx records directing to intermediate servers

[lili]

hi, 

i have a basic question on mail transfer:

when i send an email, 
my smtp server looks for the recipient smtp server in the dns (mx
record).
when will the mx record direct the emil to an intermediate smpt
server,
and when will it direct the email to the final smtp server (=so the
email will be transferred directly to it's destination, with no
intermediate stations).

are they any rules? standards? common use? 

10x a lot,
lili.