In addition to the standard SMTP specification, there is an
update called Extension to SMTP (ESMTP). Not all mail servers are
capable of using ESMTP, but it is becoming increasingly common. The
SMTP sender starts a session by issuing one of two start commands,
HELO for SMTP and EHLO for ESMTP. The type of start command
determines whether SMTP or ESMTP is used for the session. Because
some servers cannot use ESMTP, you can configure Exchange to send
the HELO command to specified domains through the SMTP
connector.
To save money, many small companies maintain transient
connections to their service providers. In addition, sometimes the
client (the SMTP sender) depends on mail arriving quickly. Forcing
the queues on the server belonging to the client's service provider
can be more desirable than waiting for a timeout to occur and the
connection to be retried. The TURN and ETRN commands allow the
client to request that a remote server start processing its mail
queues for messages to the client that are waiting at the server.
If there are messages for the client at the server, the server
sends the messages at that time.
If you choose to request remote servers to dequeue mail,
messages sent to a client through the client's service provider are
held until a TURN or ETRN command is received. The ETRN command is
more secure than the TURN command because it requests that the
server start a new connection aimed at the specified client. Thus,
the server is more likely to communicate with the correct SMTP
client.
To prevent requests on the SMTP connector for remote servers to
process the mail in their queues, select Do not send
ETRN/TURN. This option is the default.
To specify that dequeing requests are sent with the message,
select Request ETRN/TURN when sending messages. If you
choose this option, you can also request dequeuing at certain times
by selecting the Additionally request mail at specified
times check box, and then choosing dequeuing times under
Connection time.
To request dequeuing from a server other than the one to which
the message is sent, select Request ETRN/TURN from different
server, and then, under Server, type the server
name.
If you have not disabled dequeuing, to request dequeuing at
certain times, under Connection Time, select one of the
standard values in the drop-down list, or click Customize,
and then select the appropriate connection times on the timetable
grid.
To specify the ETRN or TURN command for dequeuing, under
Specify how to request that remote servers dequeue mail,
select Issue ETRN or Issue TURN. To specify the
domains for which the ETRN command should be issued, under Issue
ETRN, click Domains, and then add the domains.