RFC 5321 4.4 Trace info

Hi,

RFC 5321 4.4. Trace Information When an SMTP server receives a message 
for delivery or further processing,
it MUST insert trace ("time stamp" or "Received") information at the 
beginning of the message content,
as discussed in Section 4.1.1.4.

What is meant by "Received" ???
When looking time-stamp syntax, isn't it same thing(a bit confusing) ?

Any comments would be welcome .

Thanks,