APSMail is the library used by APS to send email, via an SMTP (Simple Mail Transfer Protocol) server, based on dynamic instructions in a mail template.
The APSMail library is designed to be called from APS; its programmatic interface is not public. However, a command line program is provided so that email templates can be sent from the command line, ostensibly for testing.
The Simple Mail Transfer Protocol (SMTP) provides a method to exchange electronic mail messages. The SMTP protocol exchange is very simple, as suggested by the protocol name. It is one of the very earliest of the internet protocols and has not changed significantly, due to its extremely wide use and installed base.
An SMTP client wishing to exchange mail with an SMTP server contacts the server on the well-known service port. The SMTP protocol is a simple ASCII, line-oriented command/response protocol. Each command is a simple command name followed by some parameters, depending on the command. Responses consist of a three-digit numeric code followed by a string explaining the response. The numeric code is easily processed by the client software and the error string can be passed on for human interpretation. Note that responses can appear on more than one line (provided the server sends the appropriate indicator).
After contacting the server, the client waits for a simple greeting message from the server. When the client receives the message, it sends a HELO (or EHLO, for Extended SMTP) command identifying itself. Further commands identify the sender and recipients and transfer the message data itself.
ESMTP, or Extended Simple Mail Transfer Protocol, is a relatively recent update to the SMTP specification that is fully backwards compatible with older SMTP clients.
APSMail communicates using RFC compliant SMTP and ESMTP, with a few optional extensions.
The original SMTP protocol is defined in RFC 821. Related RFCs include 1869 (ESMTP), 822 (Format of Electronic Mail Messages), 1521 (MIME), 1725 (POP3), and 1730 (IMAP4).
As stated above, SMTP (and ESMTP) is a line-oriented, ASCII protocol. By its very nature, it is quite slow, compared to most network protocols. While APSMail has been architected to maximize performance, this performance limitation should be taken into consideration when implementing a site.
Network latency can be significant. This is the time taken to locate and set up/tear down connections to the SMTP server.
Mail Services are designed to be distributed, a network of servers interconnected and routing information between them. APSMail is not a server/router, but rather just a client that submits messages into this network for delivery.
As such, it need not have access to specific SMTP servers, just a single server that provides an access point to the internet mail "network". In network parlance, this would be an "email gateway".
An SMTP server is a relatively lightweight service; it is for the submission of messages only, not for reading messages. As such, they do not consume large amounts of resources on the machines that they are running on. They are also relatively inexpensive, cost-wise. Microsoft Windows™ NT server, Windows 2000™ Server and Windows XP™ Server all include an SMTP server. Almost all flavors of UNIX include one as well.
To minimize network latency (and to improve fault tolerance, as discussed below), CA recommends that an SMTP server be installed and operational on the same machine where APSMail is running. In this configuration, all communications between APSMail and the SMTP server are local to the machine; no external network traffic is required. The SMTP server, after accepting the message, will communicate with the rest of the mail "network" transparently to APS. If this SMTP server will be used exclusively by APSMail then a site should configure the server to not allow access from other hosts.
SMTP servers, by their very nature, are store and forward servers. This means that it will accept messages and hold them until they can be delivered (SMTP servers are configured to retry, etc, as necessary to keep attempting delivery).
APSMail is not an SMTP service. It does not have the capability to retry sending a message if the initial send fails, that is the function of the SMTP server. A well-architected site will be designed to get the message from APSMail to the SMTP service as quickly and robustly as possible and utilize the SMTP server's own fault tolerance.
To this end, CA recommends that the SMTP service and APSMail be running on the same machine (also as mentioned above for performance). Thus, network outages, machine outages, etc, will not impact APSMail delivery.
APSMail is designed to run in either a server (multi-threaded) mode or from a command-line type mode. Each has its own resource conservation requirements. APSMail is designed to be somewhat resource conservative; that is, it makes every attempt to conserve limited machine resources, memory in particular.
This may cause slight performance degradation and creates the potential for a small fault tolerance problem.
APSMail also puts no restrictions on the number of connections to a given SMTP server. However, some SMTP servers do restrict the number of connections from a given host. This may mean that a site has to configure their SMTP service to remove this restriction. For example, if an SMTP server restricts a given client to 10 connections, but APSMail, running under a SiteMinder Policy Server, needs to send eleven pieces of mail at once, the eleventh send may fail because the SMTP server rejects the eleventh connection (there are all kinds of timing considerations that come into play). This is yet another reason to have a "private" SMTP server dedicated to each instance of APSMail.
APSMail is designed to be integrated with APS and can be configured in numerous ways. APSMail has a specific, predefined precedence system for resolving configuration conflicts.
At the highest level, APSMail can be configured using environment variables. These settings will be used unless overridden elsewhere. Most of these settings can also be made within APS. Some may also be specified or overridden within the email template files (as described later in this chapter).
This environment variable can be used to override the name of the local host as transmitted to the SMTP server in the HELO (EHLO) message. If not specified, it can be overridden in an email template file. If still not set, APSMail will use the network name of the local machine.
This environment variable specifies the IP or network name of the SMTP server. It can be overridden within the APS.cfg file and again in email template files. If not specified anywhere, mail will not be sent.
This variable specifies the email address to send to the SMTP server in the MAIL FROM command. Under the older SMTP protocol, this amounts to the login id for the SMTP server and need not match the FROM address for the message itself.
Some SMTP configurations require that this address be known to the SMTP server.
This can be overridden in an email template. If not specified at all, the FROM address of the email template will be used.
Specifies the path to an APS "send" log file, where information about sent (and rejected) mail is written. This value can be set within the APS.cfg file and from within a template. If not specified, no send log will be maintained.
This specifies a list of directories where APSMail should look for mail template files. Multiple directories can be specified. Under Windows, directories are separated by semicolons; on UNIX, directories should be separated by colons.
This value can be set in the APS.cfg file. If not specified, APSMail will not search for files; it will try to open them as configured to APS.
This environment variable specifies the name of a file where all communications between APSMail and the SMTP server(s) should be logged. Note that this file can get quite large, so it should not be used except for troubleshooting. If not specified, no communications log will be kept. This is the only way to turn on the communications log.
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