The SMTP protocol engine triggers protocol events to control the host-to-host communication. There are three main types of events that can occur in such a communication over SMTP:

• The SMTP service receives an SMTP command 
  
  These events occur when a remote SMTP host or client connects to the local SMTP service and establishes a session by sending the HELO or EHLO command. Events in this category are SMTP Protocol OnInboundCommand events on incoming connections.
  
  
• The SMTP service receives an SMTP response
  
  These events occur when the local SMTP service receives responses from a remote SMTP host or client to outbound SMTP commands. Events in this category are SMTP protocol OnServerResponse events on outbound connections.
  
  
• The SMTP service sends an SMTP command
  
  These events occur when the local SMTP service connects to a remote SMTP host and establishes a session to transfer messages. Events in this category are SMTP protocol OnSessionBegin, OnMessageStart, OnPerRecipient, OnBeforeData, and OnSessionEnd events on outbound connections.
  
  

The following table summarizes the purpose of each SMTP protocol event.

Protocol events in the SMTP service

Event	Comments
OnInboundCommand	Occurs when an SMTP command is received by the SMTP protocol service, which gives an event sink an opportunity to respond.
OnServerResponse	Occurs when the SMTP service receives an SMTP response to a previously sent SMTP command.
OnSessionBegin	Occurs before the EHLO command is transmitted.
OnMessageStart	Occurs before the MAIL FROM command is transmitted.
OnPerRecipient	Occurs before the RCPT TO command is transmitted.
OnBeforeData	Occurs before the DATA protocol command is transmitted.
OnSessionEnd	Occurs before the QUIT command is transmitted.

The Exchange Server 2007 Setup program registers Exchange-specific SMTP protocol extensions for the following SMTP protocol features.

XEXCH50

XEXCH50 is an Exchange ESMTP extension that is used to relay certain message properties, such as envelope message and recipient properties. The XEXCH50 command is a short command. An XEXCH50 command that has received a success type response is followed by a binary large object (BLOB) of variable size. (The size corresponds to the first argument of the XEXCH50 command.)

This feature is implemented using nine event sinks to support a full communication between two servers running Exchange. The following table maps the protocol events to their XEXCH50 event sinks. All XEXCH50 sinks are implemented in Peexch50.dll, which resides in the Program Files\Exchsrvr\bin directory.

Protocol extensions for the XEXCH50 command

Event sink	Protocol event	Comments
Exchange SMTP Protocol XEXCH50 Before Data Sink	OnBeforeData	Notifies the XEXCH50 sink that the DATA protocol command is about to be transmitted. The XEXCH50 sink now has the opportunity to ask the SMTP service to instead send an XEXCH50 command to start an XEXCH50 communication.
Exchange SMTP Protocol XEXCH50 Inbound EHLO Sink	OnInboundCommand	Notifies the XEXCH50 sink that an EHLO command was received.
Exchange SMTP Protocol XEXCH50 Inbound XEXCH50 Sink	OnInboundCommand	Implements the XEXCH50 command to start an XEXCH50 conversation.
Exchange SMTP Protocol XEXCH50 Inbound MAIL Sink	OnInboundCommand	Implements the MAIL command in an XEXCH50 conversation.
Exchange SMTP Protocol XEXCH50 Inbound MAIL Sink	OnInboundCommand	Enables the local SMTP virtual server to receive recipient information in an incoming XEXCH50 communication.
Exchange SMTP Protocol XEXCH50 Per Recipient Event Sink	OnPerRecipient	Enables the local SMTP virtual server to send recipient information in an outgoing XEXCH50 communication.
Exchange SMTP Protocol XEXCH50 Ehlo Response Sink	OnServerResponse	Enables the local SMTP virtual server to receive a response after an EHLO command is sent to the remote host. The response from the remote host might indicate support for XEXCH50 communications. Exchange includes XEXCH50 in the list of supported commands that are returned to the connecting host.
Exchange SMTP Protocol XEXCH50 Response Sink	OnServerResponse	Enables the local SMTP virtual server to receive a response to a previously issued outbound XEXCH50 command. For example, if the local SMTP service issued an XEXCH50 command without prior authentication, the remote server responds with: 504 Need to authenticate first.
Exchange SMTP Protocol XEXCH50 RCPT Response Sink	OnServerResponse	Enables the local SMTP virtual server to receive status information from the remote Exchange server for each recipient indicated with an outbound RCPT command. A recipient address might be incorrectly formatted or the server might be unable to relay. If the recipient information is correct, the remote SMTP virtual server reflects the address back to the local SMTP service with status information, such as: 250 2.1.5 administrator@tailspintoys.com.

X-LINK2STATE

X-LINK2STATE commands and responses are specific extensions of SMTP. This command is proprietary to Exchange, and is designed to exchange routing topology information between Exchange SMTP servers. The maximum possible size of any X-LINK2STATE command and response is 1,024 bytes. This size is frequently the actual size of X-LINK2STATE commands and responses.

This feature is implemented using five event sinks. However, one event sink is used for two separate events, as indicated in the following table. All X-LINK2STATE event sinks are implemented in Xlsasink.dll in the \Program Files\Exchsrvr\bin directory.

Protocol extensions for the X-LINK2STATE command

Event sink	Protocol event	Comments
EHLO Inbound Command Handler Sink for XLSA	OnInboundCommand	Notifies the X-LINK2STATE event sinks that an incoming EHLO command was received.
X-LSA Inbound Command Handler Sink	OnInboundCommand	Notifies the X-LINK2STATE event sinks that an incoming X-LINK2STATE command was received.
X-LSA Sink	OnMessageStart, OnSessionEnd	Signals the start (MAIL command) and end (QUIT command) of an outbound X-LINK2STATE communication. Because the remote SMTP virtual server is the ultimate recipient of the Orginfo packet being transmitted, there is no need to specify recipients in an outbound RCPT command. This event sink transmits the link state information.
X-LSA Response Handler Sink	OnServerResponse	Responds to an incoming X-LINK2STATE command with information about how to transmit the link state information. An example of a response is: 200 LAST CHUNK={00000029} MULTI (5) ({00000010} DONE_RESPONSE), which indicates the last chunk of data sent by this SMTP virtual server.
EHLO Response Handler Sink for X-LSA	OnServerResponse	Responds to an incoming EHLO command by listing the X-LINK2STATE command in the server response

X-EXPS

X-EXPS is a verb that is proprietary to Exchange, though it is similar to AUTH. There is no maximum size limit for the data chunks, or the number of data chunks exchanged. The syntax of the data commands and responses depends on the AUTH package that you select, such as LOGIN, NTLM, GSSAPI, or others. For more information, see the AUTH RFC.

Although EXPS stands for Exchange Protocol Security, the only protocol that it refers to is the SMTP. Some verbs that are used in Exchange 2000 Server and in Exchange Server 2003 are proprietary to these products and are together with ESMTP verbs. They are known as ESMTP X verbs.

These features are implemented using five event sinks, as listed in the following table. All protocol security extensions are implemented in Exps.dll in the Program Files\Exchsrvr\bin directory.

X-EXPS protocol security extensions

Event sink	Protocol event	Comments
Exchange SMTP Protocol Security EXPS-EOD Sink	OnInboundCommand	Signals the end of data transfer ( _EOD).
Exchange SMTP Protocol Security EXPS-Aux Sink	OnInboundCommand	Signals an incoming AUTH command.
Exchange SMTP Protocol Security EHLO Sink	OnInboundCommand, OnServerResponse	Signals an incoming EHLO command and responds to EHLO by listing the X-EXPS command in the server response.
Exchange SMTP Protocol Security Mail Sink	OnInboundCommand, OnServerResponse, OnMessageStart	Indicates the start of data transfer. This event sink is implemented for all relevant MAIL command scenarios. This event sink processes events that signal an incoming MAIL command, respond to an incoming MAIL command, and issue an outbound MAIL command.
Exchange SMTP Protocol Security EXPS Sink	OnInboundCommand, OnServerResponse, OnSessionStart	Indicates the start of an X-EXPS session. This event sink is implemented for all relevant X-EXPS command scenarios. This event sink processes events that signal an incoming X-EXPS command, respond to an incoming X-EXPS command, and can issue an outbound X-EXPS command.

SPAM Control

This feature is implemented using three event sinks that process sender and recipient information on incoming SMTP connections, as listed in the following table. The spam control event sinks are implemented in Turflist.dll in the Program Files\Exchsrvr\bin directory.

Spam control SMTP extensions

Event sink	Protocol event	Comments
RCPT Inbound Command Handler Sink	OnInboundCommand	Signals an inbound RCPT command with a recipient address that should be checked.
MAIL Inbound Command Handler Sink for TURF	OnInboundCommand	Signals an inbound MAIL command with a sender address that should be checked.
EOD Inbound Command Handler Sink	OnInboundCommand	Signals an inbound _EOD command.

X-ANONYMOUSTLS

This parameter is new for Exchange 2007. This parameter enables the selection of an inbound anonymous Transport Layer Security (TLS) certificate or the selection of an outbound anonymous Transport Layer Security (TLS) certificate. Exchange queries the Active Directory directory service to retrieve the thumbprint of the certificate on the server. The msExchServerInternalTLSCert attribute on the server object stores the certificate thumbprint. If the msExchServerInternalTLSCert attribute cannot be read or if the value is null, Exchange does not advertise X-ANONYMOUSTLS in the SMTP session, and no certificate is loaded.

XLONGADDR

This parameter is new for Exchange 2007. This parameter enables the Receive connector to accept long X.400 e-mail addresses. The X.400 e-mail addresses are encapsulated in SMTP e-mail addresses by using the Internet Mail Connector Encapsulated Address (IMCEA) encapsulation method.

When the value of this parameter is $False, the maximum length for a complete SMTP e-mail address is 571 characters.

When the value of this parameter is $True, the following changes are made:

• The XLONGADDR keyword is advertised in the EHLO response of the Receive connector.
  
  
• The accepted line length of an SMTP session is increased to 8,000 characters.
  
  
• Valid long addresses are accepted by the MAIL FROM: and RCPT TO: SMTP commands.
  
  

Therefore, X.400 e-mail addresses can be up to 1,860 characters long after IMCEA encapsulation.

The valid input range for this parameter is $True or $False. The default value is $False. You can only modify this parameter on Receive connectors that are configured on Hub Transport servers.

XRDST

This parameter is new for Exchange 2007. This protocol extension is used to communicate a routing destination that is associated with a message to the remote server. If a remote server does not advertise XRDST and if the message to be sent requires XRDST support, MSExchangeTransport event ID 2021 is logged in the event log. The symbolic name of this event is tuple_SmtpSendUnableToTransmitRDst. This event indicates that the message cannot be sent.

EXPS EXCHANGEAUTH GSSAPI NTLM

This parameter is new for Exchange 2007. This is a Default Receive connector services extensions that is advertised after X-ANONYMOUSTLS.

X-EXCHANGEAUTH SHA256

This parameter is new for Exchange 2007. This is a Default Receive connector services extensions that is advertised after X-ANONYMOUSTLS.