intermediate smtp servers

Hi,

I have a question:

When you send an email, your smtp server locates the smtp server of
the recipient's domain (MX record etc.)
and transfers the message to it's IP address.

The smtp server that your smtp server has located can be
a) the destination smtp server (i.e. – the final destination of the
message)
b) an intermediate smtp server that will continue transferring the
message to the destination smtp server

what's the common case ?
does case (b) ever happens ?

10x,
Lili.

p.s. – I'm not looking for an explanation on MX record look up, MX
record priorities etc. I just want to know if case (b), which
according to the smtp rfc can happen, really happens and when.
Thanks again…
Lili.