XFOR: IMAIL Conversion Scenarios in Exchange 2000 (259890)

SUMMARY

This article explains when the IMAIL conversion process takes place if a message has been submitted by a Messaging Application Programming Interface (MAPI) client and is destined for the Internet. There is a major change in the conversion process from Microsoft Exchange Server 5.5 to Microsoft Exchange 2000 Server.

This article assumes that all servers in the organization are running Exchange 2000. This article does not apply when mail is transferred to an Exchange Server 5.5 computer.

MORE INFORMATION

In Exchange 2000, content conversion is done on the server where the user submits the message. Because Simple Mail Transfer Protocol (SMTP) is the native transport for Exchange 2000, every message that leaves the server needs to be in Internet message format (SMTP). There are three possibilities, based on the type of recipient.

Scenario 1: The message is destined for an Exchange 2000 user on a server in the same routing group.

In this scenario, IMAIL renders the MAPI message into "Summary TNEF" (S-TNEF) format, a special kind of TNEF format that has no plain text part and that is rendered in binary format.
Scenario 2: The message is destined for an Exchange 2000 user on a server that is in a different routing group.

IMAIL renders the MAPI message into TNEF. This is the same TNEF format that Exchange Server uses, with a plain text part and the TNEF binary large object (BLOB) encoded in quoted printable or base-64 format, so the message can pass through a 7-bit mailer like an Exchange Server Internet Mail Service. TNEF provides this backwards compatibility. 7-bit mailers like Sendmail are not supported because Sendmail does not support the XEXCH50 SMTP extension. In Scenario 1, the message goes directly to an Exchange 2000 server, so the message uses the S-TNEF format that only Exchange 2000 understands.
Scenario 3: The message is destined for a foreign recipient (an address that is not an Exchange 2000 user in the organization).

In this instance, the Internet message format (SMTP) is chosen. IMAIL performs that rendering on the submission server, and the message leaves through SMTP. This allows the message to move from server to server by means of SMTP without further conversion, which is extremely fast. The message does not even touch the information store on subsequent servers; it simply stays within SMTP and its queue on NTFS. In Exchange Server 5.5, a message moves between servers by means of remote procedure call (RPC), preserving the MAPI properties until the Exchange Server 5.5 Internet Mail Service converts the message to SMTP format, and sends it to the Internet.