What exactly is a transport service environment which the RFC

is referring to?
Can there be a non-transport service environment? If yes,any

examples?
can we say that the originator is the first SMTP client in

transfer of mail? (And therefore does not act as an SMTP

server?)

Are there no restriction on the protocol used by a delivery

SMTP system for passing the mail to a mail user agent or

depositing it in a message store?

One part of the RFC says that relays are used when for

transporting mail between different transport environments.
"When they are
not connected to the same transport service,

transmission occurs via
one or more relay SMTP servers."
But the definition section says:
" A
"relay" SMTP system (usually referred to just as a

"relay") receives
mail from an SMTP client and transmits it, without

modification to
the message data other than adding trace information,

to another SMTP
server for further relaying or for delivery."
In other words, it does not mention that the transport

services should be different for relays to be used.

One part of the RFC says the following about gateways:
" "gateway" (that is, it may
transport the message further using some protocol other

than SMTP)."
The definition section says:
" A "gateway" SMTP system (usually referred to just as a

"gateway")
receives mail from a client system in one transport

environment and
transmits it to a server system in another transport

environment."
That is,what was said about relays earlier is now said about

gateways.

What is the true picture? Should I solely go by the

definitions? Or the things listed prior (Which do not exactly

contradict the definitions,only add to them) are the practical

meanings of those terms?

Can I call the originator as an MSA?
can I call the "delivery" SMTP system as an MDA?

What exactly is a transport service environment?
A network.

Can there be a non-transport service environment?
An isolated computer, I guess?

can we say that the originator is the first SMTP client in transfer of mail? (And therefore does not act as an SMTP server?)
It would depend on how the network is set up, but yes, the first SMTP link needn't receive any SMTP requests.

Are there no restriction on the protocol used by a delivery SMTP system for passing the mail to a mail user agent or depositing it in a message store?
That's really outside of the scope of the RFC. We might have an agreement: you give me milk for money. What I do with the milk doesn't affect our exchange, nor does how you got the milk.

In other words, it does not mention that the transport services should be different for relays to be used.
True, but you typically see relays on the "edges" of private networks; they receive an email from the public/private side, and they forward it to the private/public side.

That is,what was said about relays earlier is now said about gateways.
My previous example would typically be referred to as a gateway, as it's the entrance/exit to a private network. Basically, a relay need not be a gateway, but a gateway is probably a relay. It's a situation caused by people creating and using terms, then assigning definitions after the fact.

Can I call the originator as an MSA?
can I call the "delivery" SMTP system as an MDA?
I guess you could, but I have no idea what you're talking about.

'tis the season for homework again, if you had GOOGLED would of found lots of info, maybe this:

http://tools.ietf.org/html/rfc2821
or
http://en.wikipedia.org/wiki/Simple...