With the addition of the SMTP/POP3 gateway for EMS, EMS customers are able to, for
the first time, connect directly or indirectly to the internet or an intranet, and
exchange mail with other SMTP compliant mail systems. This includes UNIX systems
as well as Microsoft Exchange Server and Lotus Notes Server. It also allows for
POP3 based clients such as Microsoft’s Outlook or Outlook Express, or Netscape’s
Navigator, or Qualcomm’s Eudora, to name a few, to use EMS as their mail server
either over the internet, or within a company’s internal intranet.
The headaches that used to accompany attempts to communicate between other companies
are no longer there. No more address translations as mail is passed from gateway
to gateway and through value added networks, just to get from point A to point B.
All that is needed are simple internet SMTP addresses.
No more forcing your company employees that do not use other 3270 applications to
use a 3270 interface for their mail system, or, worse yet, to manage multiple mail
systems just so they can have GUI based software on their workstations. Now EMS
can service both your 3270 users and your GUI users. And, best of all, you’re not
stuck with a proprietary mail system for your users to learn. As mentioned, any
software product that supports POP3 mail exchange is able to connect to EMS as its
Overview of SMTP
SMTP, Simple Mail Transfer Protocol, is a standard for electronic mail interchange.
The standard was developed and set by the developers of the internet. SMTP was developed
predominately on UNIX; however, it has become a widely supported cross platform
standard for mail exchange. SMTP is considered a part of the suite of standard applications
or services included in a full function TCP/IP software system (often called the
"Stack"). Some other features are FTP (File Transfer Protocol) and HTTP,
which is the basis for Web Browsers.
The standard SMTP facility supplied with IBM's TCP/IP stack allows TSO users
to send and receive mail, and uses JES as a repository for inbound and outbound
messages. EMS's SMTP gateway replaces the IBM SMTP process with a process that
runs under CICS. Inbound and outbound messages are kept in VSAM files and delivered
to either EMS users with 3270 interfaces or POP3 clients, such as MS Exchange, Netscape
SMTP was defined to only carry ASCII text. However, it has been supplemented by
the MIME standard that supports the encoding of attached binary files. Since an
SMTP message may pass through many machines before final delivery, a method called
BASE64 encoding is used to transform a binary file into an all text file using an
algorithm that only uses characters supported by all computers.
Email addresses in SMTP are based around a two part address: the "local part",
usually a user ID or name, and the "domain part" separated by an "at"
(@) sign . For example: "firstname.lastname@example.org". In the world of the internet,
the domain names are registered with the internet Network Information Center (INTER-NIC).
An EMS user can address a message inline, on the fly, or with a redefined EMS shadow
profile. For on the fly addressing, a message could begin with "/TO /SMTP email@example.com".
If SMTP has been defined as the "default" external gateway, the message
author only needs "/TO firstname.lastname@example.org" and EMS will know that the message
is for the SMTP gateway.
SMTP addresses can also be kept with the personal nicknames, on the master name
directory, or in an EMS profile that is explicitly set up as an SMTP destination.
When messages are received from an SMTP source, EMS keeps the full return address
and the user can amend and reply directly to the message without having to rekey
Overview of POP3
Whereas SMTP is sometimes called a "Push" protocol--it pushes the message
to its next destination and assumes that a user at the destination will "signon"
and read their mail directly-- POP3 is a "Pull" protocol, in that the
user's mail client pulls down the message to their work station. In other words,
a POP3 client will signon to your mailbox, download all of your unread mail, and
disconnect, allowing you to work offline on your PC. POP3 is a protocol defined
in the internet standards, and as such has been implemented on many platforms. EMS's
POP3 server facility allows POP3 clients to pick up their mail from EMS without
ever seeing a mainframe (3270) screen image. When the POP3 client/user composes
messages to send, the client agent connects to the SMTP gateway of EMS and accepts
and then sends their mail.
EMS's POP3 server also supports MIME (binary) attachments, and will even download
attached EMS reports.
EMS has tested the POP3 interface with Microsoft's mail exchange client, which
is bundled with Windows 95 and NT, Netscape Navigator, Eudora Mail Pro, and Z-Mail
Pro from Net Manage. When moving a message with an attachment across EMS, we even
keep the original filename and suffix, so the receiver of the attached "spread
sheet" (or whatever) can simply click on "attachment" and launch
the appropriate application.
With the SMTP gateway and POP3 server, EMS becomes a super size email server with
all of the reliability and scale-ability of the mainframe. Instead of a mail server
supporting a few hundred users, some EMS customers are supporting several thousand
users on a single system.
TCP/IP on the mainframe
For many years, the main communication protocol supported by IBM on the mainframe
was its own proprietary SNA. TCP/IP (Transmission Control Protocol/Internet Protocol)
is an alternative communication protocol. The standard "suite" is based
on ASCII, not EBCDIC, and includes some higher level services such as email, file
transfer, remote login, and telnet including TN3270. The "suite" of services
is still growing. There are many books on the TCP/IP subject, so we won't try
to reproduce them here.
We have found three TCP/IP stacks for IBM mainframes. The first is from IBM and
is for MVS, the second is from Interlink and is also for MVS, and the third is for
The TCP/IP stack from IBM includes a program interface called CICS-Sockets which
EMS uses for the SMTP and POP3 facilities. EMS's SMTP gateway replaces the SMTP
task supplied with IBM's TCP/IP stack. In the case of POP3, IBM does not supply
a POP3 server.
Internet vs Intranet
Two of the biggest "buzz words" going around are internet and intranet.
For the purposes of this document we will assume that both are basically just TCP/IP
networks, and the main difference is that internet connects your system to the world
wide network of networked computers, and an intranet uses the TCP/IP protocol to
connect computers within your organization. You may have both, an intranet for your
in-house computers and workstations, and access points or gateways or "firewalls"
to connect you to the outside world. Basically, you need to talk to an ISP (Internet
Service Provider) about connection to the internet.
With TCP/IP on one end and a PC with TCP/IP on the other end ... mainframe to PC
integration is pretty tight. With the TN3270 access the user can access the host's
"legacy", or core business, systems, and EMS supplies the email to the
user's preferred POP3 mail client.
Remote users -- Traveling
Since the POP3 client "pulls down" the mail, a person traveling can connect
to a local ISP, connect to their mailbox in EMS, upload any outbound mail, download
any new mail, and disconnect.
Standard remote client
One very nice feature of the EMS SMTP/POP3 setup is that there is no special client
software to distribute, license, track, or support. The TCP/IP and Mail agents come
with Windows or are available off-the-shelf for a very low cost. Basically, we do
the mainframe software and Microsoft (or others) do the PC software.
PassThru vs Break Apart -- Attachment handling
When EMS receives an SMTP message that has several attachments, it normally breaks
the various attachments into separate EMS messages. This allows a message that comes
in from the internet with an attached spread sheet file to be downloaded by the
3270 user with a tool such as IND$FILE, or maybe the message will be forwarded to
an MHS destination.
This is fine for an EMS 3270 user, and is usually ok for a POP3 user in the same
situation. However, as internet messages begin to contain multiple attachments,
including an HTML "package" that is intended to be viewed with a browser,
this approach has limitations.
With the PassThru option, messages that are received from an I-net can be passed
through EMS with few or no changes; what comes in goes out. With this approach,
messages are not broken apart, and the entire received message is "packaged
together" as a single attached report and shipped to its destination.
To enable the PassThru facility in general, a setting on the SMTP Gateway Default
settings screens must be enabled.
Second, when the message is received, it looks at where it is going. If it is going
directly out to an SMTP destination, then it is just passed through. This most commonly
occurs when a POP3 client sends an outbound message; it is uploaded to EMS which
in turn ships it out to the internet.
The second situation that triggers the PassThru is when the destination is an EMS
user that has requested that their message be passed through. This is most likely
an EMS user that uses a POP3 client to pickup their mail and generally never uses
the 3270 interface. This "Passthru" mode is set in the #2 screen of the
user's EMS profile.
The "Allow POP3 Access" should only be set to "P" if the user
is normally a POP3 user and wants all of their mail be passed through with minimal
The following diagrams exhibit various possible connection configurations for the
EMS SMTP/POP facility. They may be helpful to understand where the various elements
fit in the scheme of things.
Direct SMTP to/from Internet
Figure 1 illustrates a configuration in which the Host is talking directly to the
internet. In this situation, the Host must stay UP all of the time to accept incoming
messages. Well, not really "all of the time" since the SMTP "senders"
are going to have "retry" logic if the host goes down for a short period
of time. In this setup, EMS would attempt to deliver each message to a target Host
that will accept the mail item. The registered Domain Name points to the IP address
of the MVS system.
If EMS cannot find a target HOST to accept the mail, the item will most likely be
"passed" off to your ISP's postoffice/mailer to attempt regular message
delivery. Since any number of "remote senders" could be trying to deliver
a message to us, there could be several receive processes going on at the same time.
At present, the system is configured to support 50 active receivers.
- Most direct message delivery. EMS can be confident that the message was delivered
or not. No extra message "hops".
- No need for UNIX knowledge.
- Host must stay up to receive mail.
- Could have to support unknown number of incoming "receiver" tasks.
SMTP to/from Internet w/UNIX
The configuration depicted in Figure 2 assumes that a UNIX machine will act as a
buffer between the mainframe and the internet. We tested the system with a SCO/UNIX
system, but Windows NT with proper mailer software should do the job as well.
This configuration assumes that the Registered Domain name points to the IP address
of the UNIX box.
The MAILER on the UNIX box is configured to accept all incoming mail and forward
it to the MVS/EMS system. In this manner the UNIX box acts as a receiver postoffice/buffer
and should the mainframe go down, mail can still be received on the UNIX system.
Since only the UNIX box should be sending mail to the MVS/EMS system, this should
keep the number of "receiver" tasks to one.
This is a configuration whereby the UNIX box could be located outside of the "firewall"
and the MVS system is protected inside the "firewall".
Outbound mail can be handled in one of two ways. The first is to simply dump all
of the outbound mail to the UNIX box and let it route/deliver the mail. The disadvantage
here is that the only delivery process that EMS knows about is to the UNIX box,
which will always work, and we won't know if the message failed to be delivered
until the UNIX system attempts delivery and sends back a "notice of failure"
after the fact. But there are several advantages to this setup: the UNIX box can
be outside the firewall and the connection to the UNIX box is very fast, thus we
can pass off the outbound mail traffic very quickly.
The second is for EMS to attempt direct delivery, and then only mail that cannot
be routed is dumped to the UNIX box as a backup. This has the advantage of EMS knowing
if the message was delivered properly or explicitly rejected, and only mail without
a proper DNS lookup or a valid domain name is passed to the UNIX system.
It should be noted that the communications protocol is still TCP/IP from the MVS/CICS/EMS
system to the UNIX box.
POP3 with no Internet
Figure 3 shows a configuration with no internet access, thus it is truly an intranet
In this configuration we don't even have internet access. Why would anyone want
- We want the 3270 users and the PC users to have email
- We don't want to manage multiple mail servers
- We don't want to install custom software on each PC
The POP3 (read PC) user never even sees a 3270 screen. Their POP3 mailer connects
to the mainframe and downloads their mail, including attachments (binary or not).
When they send mail, the PC client passes the mail to the SMTP facility on the MVS/EMS
mainframe. In effect, the PC just treats the mainframe like just another (big) server.
In the diagram we have included Macintosh computers. We have not personally tested
a Mac (we don't have any), but as long as the "client" is POP3/SMTP
compliant, it should work.
POP3 and Internet
The configuration shown in Figure 4 includes everything. It assumes that the UNIX
box is used as a buffer and the MVS/EMS box ALSO has direct internet access. The
Laptop user in this case can dial into a local internet provider and have the POP3
client on his laptop connect to EMS on the MVS system, signon, pickup mail, and
disconnect. At the same time, any outbound "queued" up mail is passed
up to EMS for delivery.