Storing and Forwarding With SMTP And Message Transfer Agents

Gateways: 2000 and Beyond! Other wrinkles in an e-mail message's daily routine are gateways. For example, your company uses Microsoft Exchange, cc:Mail, or Novell GroupWise as your mail system. When a user within these systems wishes to send mail to a person on the Internet, a gateway must be installed. The gateway acts as a specialized MTA and converts the mail message from the internal mail format (MS Mail, Message Handling System, etc.ý) to a form acceptable to Internet MTAs. In our discussion of SMTP, we did not discuss how non-SMTP based messaging systems would converse over the Internet. When following the evolution of an enterprise messaging system, you will often see an internal messaging system predate an external-internal system. This pattern is easily explained when considering the need for collaborative software such as MS Exchange, Lotus Notes and Novell GroupWise software packages . Corporations that grasp the potential of a computer network will look to develop these systems before more complicated issues such as Internet mail, Web serving and file systems. Each of these systems can be configured with accompanying messaging software. MS Mail, cc:Mail and Novell's Message Handling System all use non-SMTP transport mechanisms. Integrating these proprietary systems with the open-standards-based Internet is easily accomplished with the use of an SMTP gateway. The gateway is nothing more than a specialized MTA designed to interpret and translate the internal addressing scheme to one acceptable to the Internet. The translation of the internal addressing scheme is done at the gateway MTA, and when it is set up correctly it is completely transparent to end users. In addition to translating the addresses from one format to another, the message content must be transmittable over the SMTP protocol. For example, X.400 messages can include non-ASCII content that cannot be sent over SMTP. When an outgoing message containing non-ASCII content arrives at the SMTP gateway, it must be converted to a MIME attachment. Once the message is converted, it can be sent over the Internet via SMTP. Many non-SMTP-based messaging systems use non-ASCII character sets. In order to facilitate the transport of message content over SMTP-based MTAs on the Internet, all SMTP gateways should use a common character set when converting content. This common character set includes 'a-z', 'A-Z', '0-9' and the following 11 special characters: "'" (ASCII code 39) "(" (ASCII code 40) ")" (ASCII code 41) "+" (ASCII code 43) "," (ASCII code 44) "-" (ASCII code 45) "." (ASCII code 46) "/" (ASCII code 47) ":" (ASCII code 58) "=" (ASCII code 61) "?" (ASCII code 63) Naturally the most common and interoperable encoding mechanisms restrict themselves to these 73 characters. Base64 and uuencode both adhere to this restriction. Another problem with SMTP gateways is mistak en interpretation of corrupt message content. For example you have written a message that contains a single '.' on a blank line. Normally this would signal the end of the message and any message content after it would be interpreted as SMTP commands in the SMTP dialog. To avoid this problem, a properly implemented MUA will encode the '.' as '=2E' before transporting the message to the MTA. The '2E' is a proper replacement as it is the ASCII code of the '.'. A similar situation involves a line beginning with "FROM," which would normally indicate the beginning of a message. "FROM " would be substituted with "=46ROM." It is NOT recommended for an MTA to alter the composition of a message and specifically change white space. Running a few test messages through your MTA will indicate if it is changing message content or not. This is an important issue and could also influence your mail client purchase decision. If you are adding Internet messaging services to your existing network, you should definitely use an SMTP gateway in conjunction with legacy messaging systems. Once the gateway is in place, there are some configuration issues worth examining. The address translation is not very difficult to understand, however the addresses can be confusing. Each old user on the legacy system has a corresponding user name on the gateway. Most times the same user name can be used--however it may make more sense to create new user names that are less complicated. For example, your internal MS Mail user name may be 'John.Doe.' You may wish to make the Internet mailing address something less complex like 'jdoe.' The less complicated the mailing address, the easier it is to share with others over the Internet. Short, succinct user names will be easy for others to remember. Print This Page E-mail this URL