SMTP Protocol Overview

[kiran2710]

SMTP Protocol Overview

Simple Mail Transfer Protocol (SMTP), documented in RFC 821, is Internet's standard host-to-host mail transport protocol and traditionally operates over TCP, port 25. In other words, a UNIX user can type telnet hostname 25 and connect with an SMTP server, if one is present.
SMTP uses a style of asymmetric request-response protocol popular in the early 1980s, and still seen occasionally, most often in mail protocols. The protocol is designed to be equally useful to either a computer or a human, though not too forgiving of the human. From the server's viewpoint, a clear set of commands is provided and well-documented in the RFC. For the human, all the commands are clearly terminated by newlines and a HELP command lists all of them. From the sender's viewpoint, the command replies always take the form of text lines, each starting with a three-digit code identifying the result of the operation, a continuation character to indicate another lines following, and then arbitrary text information designed to be informative to a human.
If mail delivery fails, sendmail (the most important SMTP implementation) will queue mail messages and retry delivery later. However, a backoff algorithm is used, and no mechanism exists to poll all Internet hosts for mail, nor does SMTP provide any mailbox facility, or any special features beyond mail transport. For these reasons, SMTP isn't a good choice for hosts situated behind highly unpredictable lines (like modems). A better-connected host can be designated as a DNS mail exchanger, then arrange for a relay scheme. Currently, there two main configurations that can be used. One is to configure POP mailboxes and a POP server on the exchange host, and let all users use POP-enabled mail clients. The other possibility is to arrange for a periodic SMTP mail transfer from the exchange host to another, local SMTP exchange host which has been queuing all the outbound mail. Of course, since this solution does not allow full-time Internet access, it is not too preferred.
RFC 1869 defined the capability for SMTP service extensions, creating Extended SMTP, or ESMTP. ESMTP is by definition extensible, allowing new service extensions to be defined and registered with IANA. Probably the most important extension currently available is Delivery Status Notification (DSN), defined in RFC 1891.

SMTP Gateway Version 1.0 - Concepts




The SMTP Gateway consists of two parts:
SMTPGATE.EXE runs on a Windows NT Workstation or Server.
SEGCONFIG.EXE is a configuration utility for the Gateway. The Gateway configuration can be completed when the Gateway is running or when it is shutdown. The configuration utility communicates with the Gateway using a protocol called RPC. (Remote Procedure Call)






SMTPGATE.EXE<--------------RP----------------->SMTPCONFIG.



·E-mail relies on a client and server connection whether you use Lotus Notes, MS Exchange, Exchange or an Internet mail client communicating with your ISP’s mail server.

·SMTP is a push/pull protocol over TCP/IP usually on port 25. The Castelle SMTP Gateway must communicate via SMTP to your SMTP server, not the client (although some “pure” SMTP clients like Eudora may communicate directly with the SMTP Gateway). The port is important to remember when configuring the Gateway. The FaxPress SMTP Gateway cannot reside on the same server as the SMTP Mail Server, because both will need to listen on the same port (25).

E-mail Client.
E.g. MS -------------->
E-mail Server.
E.g. MS

SMTP
Port 25
<-----------> Castelle FaxPress










Faxes Sent Via the SMTP Gateway
Sending a Faxmail from the e-mail client (Exchange Client, Outlook, Etc.) will require an entry in the “Send To” field. Assuming the SMTP Gateway is installed on a system with IP address 204.23.218.40, (which has a DNS address of fax.company.com) the entry on the “Send to” line would read:

1. User sends e-mail to phone_no@address.dom
(e.g. 9169398899@fax.company.com or
9169398899@204.23.218.40 )

2. The e-mail message is sent via SMTP from the mail host to the SMTP Gateway system. If the user is using a 'pure' SMTP client like Eudora, the user’s workstation contacts SMTPGate directly using SMTP. If the user is using Exchange, it travels first to the Exchange server, then Exchange routes it to its Internet Message Transfer Agent (MTA) which then sends it to SMTPGate using SMTP. Either way, SMTPGate communicates with the 'sender' using SMTP. The Gateway does not know if it is talking to an Exchange Server, GroupWise Server, cc:Mail Server, or any other SMTP Mail Server.

3. SMTP Gateway converts e-mail messages and attachments sending them to the FaxPress. The recipient information (Name, Phone Number and Message) is then merged to the cover page. This process is just as if you had sat at the SMTP Gateway machine and sent the fax from Faxmain. (This is why we ask users to send a successful fax for Faxmain on the SMTP Gateway system before trying to send FaxMail from the SMTP Gateway).

4. After the fax has been sent (or fails to send), the SMTPGate sends an e-mail back to the user telling them of the results.

How Received Faxes Are Processed

Faxes are received by FaxPress and routed in two ways.

1.The faxes are received at the unaddressed mailbox and manually routed to the user’s mailbox.

2.The best way to rout faxes is automatically with DID (Direct Inward Dialing). Assigned phone numbers are given to each fax user. FaxPress receives the tones to route it into a given users inbox. If not successful it travels to unaddressed mailbox.

SMTP Gateway periodically polls the FaxPress mailboxes for received faxes. If a fax is received, the SMTP Gateway checks the User Map to match the FaxPress user to their e-mail address. The information is packaged into an e-mail message, and e-mails it to the specified user.