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The Basics

There are many different e-mail systems available for your network, but to describe how SMTP works, we'll concentrate on a standard MUA/MTA setup.

When a Mail User Agent (MUA) wishes to send mail across the Internet, it first passes mail to another machine called the Mail Transport Agent (MTA). This MTA communicates with other MTAs on the Internet using SMTP.

SMTP is the protocol for transporting mail over the Internet.

SMTP is spoken via port 25 of the MTAs. Using the basic model described above, a session is performed as follows:

1. The local MTA establishes a connection with the remote MTA. (They exchange 'hellos.')

2. The local MTA informs the remote MTA that it has mail from a local user. The remote machine acknowledges.

3. The local MTA informs the remote MTA to whom the mail is addressed on the remote side. The remote machine acknowledges.

4. The local MTA sends the message and the remote machine acknowledges its arrival.

5. The machines break connection.

The abstract session described above shows the basic concepts of an SMTP transaction.

SMTP Gateways

Depending on what type of network you have established on your side and what mail software is running, you will most likely need to have:

• An SMTP gateway on your network
  or
• Access to a machine that can perform these SMTP transactions

An SMTP gateway allows users on your network to communicate with others on the Internet without concern as to which local mail software package exists on your network.

All incoming mail for your network will pass through this gateway which converts the message into the appropriate format specific to your local mail software.

Similarly, all mail destined for the Internet from your network will pass through this gateway to be sent across the Internet via SMTP.

SMTP Relays

An SMTP relay is a machine that actually sends the mail across the Internet.

A common misconception is that SMTP gateways are the same as SMTP relays. This is not always the case.

There are SMTP gateways that act as relays themselves, but there are also many that do not. If the latter is the case on your network, you'll need to bounce your mail off one of the relays.

So what's the difference?

SMTP gateways convert your mail messages into the proper form for transfer across the Internet. This does not imply that the gateway is smart enough to do things such as DNS translations. SMTP relays accomplish such tasks.