Microsoft reduced the number of server roles in Exchange 2013 to
just two in order to “increase simplicity of scale, hardware utilization and
failure isolation”:
Mailbox Server role:
which includes the Client Access protocols, Hub Transport
service, Mailbox databases and Unified Messaging, and also handles all activity
for a given mailbox.
Client Access Server role:
which provides authentication, redirection and proxy services
(it doesn't do any data rendering). The Client Access server is a thin and
stateless server and there is never anything queued or stored on it. The Client
Access server offers the usual client access protocols: HTTP (Outlook Anywhere,
ActiveSync and Web Services), POP, IMAP and SMTP, but not MAPI (all MAPI connections
are encapsulated using RPC-over-HTTPS)!
Note that the Edge Server is not present in Exchange 2013. This
is because its services are now provided by the Client Access Server (although
you can still use an Exchange 2007/2010 Edge server as a gateway with your
Exchange 2013 environment).
In this new architecture, the Client Access server and the
Mailbox server roles are not as dependent on one another as in previous version
of Exchange because all processing and activity for mailboxes occurs on the Mailbox
server where the active database copy of a mailbox resides. All data rendering
and data transformation is performed local to the active database copy,
eliminating concerns of version compatibility between the Client Access server
and the Mailbox server.
Transport Pipeline:
Mail flow takes place through the Transport Pipeline which is a
collection of services, connections, components and queues that work together
to route messages. With Exchange 2013, the transport pipeline is now made up of
3 different services:
1. Front End Transport service
which runs on all Client Access servers, acts as a stateless
proxy for all inbound and outbound external SMTP traffic. This service does not
inspect message content but it can perform basic e-mail filtering based on connections,
domains, senders and recipients. It only communicates with the Transport
service on a Mailbox server and does not queue any messages locally.
2. Transport service
runs on all
Mailbox servers and is almost identical to the Hub Transport server role in
previous versions of Exchange. It handles all SMTP mail flow for the
organization, performs message categorization and content inspection. Unlike
previous versions of Exchange, the Transport service never communicates
directly with a mailbox database, which is now handled by the Mailbox Transport
service. The Transport service routes messages between the Mailbox Transport
service, the Transport service and the Front End Transport service. This
service does queue messages locally.
3. Mailbox Transport service
also runs on all Mailbox servers and is made of two separate
services:
a. The Mailbox Transport Delivery service
receives SMTP messages from the Transport
service and connects to the mailbox database using an Exchange Remote Procedure
Call [RPC] to deliver the message.
b. The Mailbox Transport Submission service
connects to the mailbox database using
RPC to retrieve messages and submits them over SMTP to the Transport service.
The Mailbox Transport service also does not queue any messages locally.
As Exchange 2013 no longer has an Edge Server role, e-mail
messages from the Internet are received and sent to Exchange using a
third-party e-mail gateway, an Exchange 2007/2010 Edge server or through the
Exchange 2013 Client Access server as Microsoft intends it to be. In this last
scenario, these e-mails enter the Exchange organization through a Receive
connector in the Front End Transport service and are then routed to the
Transport service on a Mailbox server.
While Exchange 2007/2010 Hub Transport servers were not
configured out of the box to accept e-mails from the internet, the new Client
Access server comes with a Receive Connector named “Default Front end
<server_name>” that is already configured to allow “Anonymous Users” to
connect to it:
Transport service on Edge Transport servers
This service is very similar to the Transport service on Mailbox servers. If you have an Edge Transport server installed in the perimeter network, all mail coming from the Internet or going to the Internet flows through the Transport service Edge Transport server.
The following figure shows the relationships among the
components in the Exchange 2013 transport pipeline.
Mail flow documentation:
Topic
|
Description
|
Mail routing describes how messages are
transmitted between messaging servers.
|
|
Connectors define where and how messages
are transmitted to and from Exchange servers.
|
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Accepted domains define the SMTP address
spaces that are used in the Exchange organization. Remote domains configure
message formatting and encoding settings for messages sent to external
domains.
|
|
Transport agents act on messages as they
travel through the Exchange transport pipeline.
|
|
Transport high availability describes how
Exchange 2013 keeps redundant copies of messages during transit and after
delivery.
|
|
Transport logs record what happens to
messages as they flow through the transport pipeline.
|
|
Moderated transport requires approval for
messages sent to specific recipients.
|
|
Content conversion controls the Transport
Neutral encoding format (TNEF) message conversion options for external
recipients, and the MAPI conversion options for internal recipients.
|
|
Delivery status notifications (DSNs) are
the system messages that are sent to message senders, for example,
non-delivery reports (NDRs).
|
|
Delivery Reports is a message tracking
tool that you can use to search for delivery status on email messages sent to
or from users in your organization's address book, with a certain subject.
You can track delivery information about messages sent by or received from
any specific mailbox in your organization.
|
|
This topic describes the size and
individual component limits that are imposed on messages.
|
|
You use the Queue Viewer in the Exchange
Toolbox to view and act upon queues and message in queues.
|
|
The pickup and replay directories are
used to insert message files into the transport pipeline.
|
|
This topic describes the considerations
for using an Edge Transport server from previous versions of Exchange in
Exchange 2013.
|
Messages from external senders
Messages from outside the organization enter the transport
pipeline through a Receive connector in the Front End Transport service on the
Client Access server and are then routed to the Transport service on the
Mailbox server.
If you have an Exchange 2013 Edge Transport server installed in
the perimeter network, messages from outside the organization enter the transport
pipeline through a Receive connector in the Transport service on the Edge
Transport server. Where the messages go next depends on how your internal
Exchange servers are configured.
·
Mailbox server and
Client Access server installed on the same computer In this configuration, the Client Access
server is used for inbound mail flow. Mail flows from the Transport service on
the Edge Transport server to the Front End Transport service on the Client
Access server, and then to the Transport service on the Mailbox server.
·
Mailbox server and
Client Access server installed on different computers In this configuration, the Client Access
server is bypassed for inbound mail flow. Mail flows from the Transport service
on the Edge Transport server to the Transport service on the Mailbox server.
Messages from internal senders
SMTP messages from inside the organization enter the transport
pipeline through the Transport service on a Mailbox server in one of the
following ways:
·
Through a Receive
connector.
·
From the Pickup
directory or the Replay directory.
·
From the Mailbox
Transport service.
·
Through agent
submission.
The message is routed based on the routing destination or delivery
group. If the message has external recipients, the message is routed from the
Transport service on the Mailbox server to the Internet, or from the Mailbox
server to the Front End Transport service on a Client Access server and then to
the Internet if the Send connector is configured to proxy outbound connections
through the Client Access server.
If you have an Edge Transport server installed in the perimeter
network, messages that have external recipients are never routed through the
Front End Transport service on a Client Access server. The message is routed
from the Transport service on a Mailbox server to the Transport service on the
Edge Transport server.
Transport service on Mailbox servers
Every message that's sent or
received in an Exchange 2013 organization must be categorized in the Transport
service on a Mailbox server before it can be routed and delivered. After a
message has been categorized, it's put in a delivery queue for delivery to the
destination mailbox database, the destination database availability group
(DAG), Active Directory site, or Active Directory forest, or to the destination
domain outside the organization.
The Transport service on a Mailbox server consists of the
following components and processes:
·
SMTP Receive When messages are received by the Transport
service, message content inspection is performed, transport rules are applied,
and anti-spam and anti-malware inspection is performed if they are enabled. The
SMTP session has a series of events that work together in a specific order to
validate the contents of a message before it's accepted. After a message has
passed completely through SMTP Receive and isn't rejected by receive events, or
by an anti-spam and anti-malware agent, it's put in the Submission queue.
·
Submission Submission is the process of putting messages
into the Submission queue. The categorizer picks up one message at a time for
categorization. Submission happens in three ways:
o From SMTP Receive through a Receive connector.
o Through the Pickup directory or the Replay directory. These
directories exist on Mailbox servers and Edge Transport servers. Correctly
formatted message files that are copied into the Pickup directory or the Replay
directory are put directly into the Submission queue.
o Through a transport agent.
·
Categorizer The categorizer picks up one message at a time
from the Submission queue. The categorizer completes the following steps:
o Recipient resolution, which includes top-level addressing,
expansion, and bifurcation.
o Routing resolution.
o Content conversion.
Additionally,
mail flow rules that are defined by the organization are applied. After
messages have been categorized, they're put into a delivery queue that's based
on the destination of the message. Messages are queued by the destination
mailbox database, DAG, Active Directory site, Active Directory forest or
external domain.
·
SMTP Send How messages are routed from the Transport
service depends on the location of the message recipients relative to the
Mailbox server where categorization occurred. The message could be routed to
one of the following locations:
o To the Mailbox Transport service on the same Mailbox server.
o To the Mailbox Transport service on a different Mailbox server
that's part of the same DAG.
o To the Transport service on a Mailbox server in a different DAG,
Active Directory site, or Active Directory forest.
o For delivery to the Internet through a Send connector on the
same Mailbox server, through the Transport service on a different Mailbox
server, through the Front End Transport service on a Client Access server, or
through the Transport service on an Edge Transport server in the perimeter
network.
Transport service on Edge
Transport servers
The
components of the Transport service on Edge Transport servers are identical to
the components of the Transport service on Mailbox servers. However, what actually
happens during each stage of processing on Edge Transport servers is different.
The differences are described in the following list.
- · SMTP Receive When an Edge Transport server is subscribed to an internal Active Directory site, the default Receive connector is automatically configured to accept mail from internal Mailbox servers and from the Internet. When Internet messages arrive at the Edge Transport server, anti-spam agents filter connections and message contents, and help identify the sender and the recipient while the message is being accepted into the organization. The anti-spam agents are installed and enabled by default. Additional attachment filtering and connection filtering features are available, but built-in malware filtering is not. Also, transport rules are controlled by the Edge Rule agent. Compared to the Transport Rule agent on Mailbox servers, only a small subset of transport rule conditions are available on Edge Transport servers. But, there are unique transport rule actions related to SMTP connections that are available only on Edge Transport servers.
- · Submission On an Edge Transport server, messages typically enter the Submission queue through a Receive connector. However, the Pickup directory and the Replay directory are also available.
- · Categorizer On an Edge Transport server, categorization is a short process in which the message is put directly into a delivery queue for delivery to internal or external recipients.
- SMTP Send When an Edge Transport server is subscribed to an internal Active Directory site, two Send connectors are automatically created and configured. One is responsible for sending outbound mail to Internet recipients; the other is responsible for sending inbound mail from the Internet to internal recipients. Inbound mail is sent to the Transport service on an available Mailbox server in the subscribed Active Directory site.
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