inbound (Receive) mail flow?
1) - The sending SMTP server queries Domain Name System (DNS) to locate the mail exchanger (MX) resource record of the recipient's SMTP mail server. This MX record resolves to a corresponding host (A) record that resolves the IP address of the recipient's SMTP mail server.
2) - The sending SMTP server initiates a conversation on the recipient's SMTP server (using port 25). On an Exchange Server gateway, the recipient's SMTP server is the SMTP virtual server on the Exchange server that is configured to accept inbound mail.
3) - If the message is destined for a recipient of its SMTP mail domain, the SMTP server accepts the inbound message, as defined by recipient policies.
4) - When the message is accepted, the message is persisted in the \Queue folder on the Exchange server. The SMTP virtual server submits the message to the Advanced Queuing Engine, which then submits the message to the message categorizer.
5) - The message categorizer validates the recipients of the message, checks for proper recipient attributes, applies limits and restrictions, flags the message for local delivery, and then returns the message to the Advanced Queuing Engine.
6) - The Advanced Queuing Engine submits the message to the Local Delivery queue.
7) - The Exchange store receives the message from the Local Delivery queue.
8) - Mail messages are delivered to the client (for example Outlook, Outlook Express, or Outlook Web Access).
Outbound mail flows through an Exchange Server deployment in the following manner:
1) Mail messages are sent from a client (Microsoft Outlook, Outlook Express, or Outlook Web Access, for example) and are submitted to the local Exchange store.
2) The Exchange store submits the message to the Advanced Queuing Engine.
3) The Advanced Queuing Engine submits the message to the message categorizer.
4) The message categorizer validates the recipients of the message, checks for proper recipient attributes, applies limits and restrictions, flags the message for local or remote delivery, and then returns the message to the Advanced Queuing Engine.
5) If for local delivery, the Advanced Queuing Engine submits the message to the Local Delivery queue, and the Exchange store receives the message from the Local Delivery queue.
6) If for remote delivery, the Advanced Queuing Engine submits the message to the Routing Engine. The Routing Engine determines the most efficient route for mail delivery, returns the message to the Advanced Queuing Engine, and, in turn, submits the messages for remote delivery. The messages are then sent via SMTP to a remote SMTP host or to the Internet.
* External DNS Queries work?
When a DNS client needs to resolve the name of a server, it queries the DNS servers. Each query that the client sends essentially asks the DNS server to provide the information. The client specifies the query type, which can either indicate a resource record by type or a specialized type of query operation. For example, to find SMTP mail servers from the Internet, specify the query type MX (mail exchanger resource record).
For example, the name that is specified could be an external domain, such as example.microsoft.com., and the query type that is specified to look for could be an MX record by that name. Think of a DNS query as a client asking a server a two-part question: First, "Do you have any MX resource records for a domain named 'example.microsoft.com.'?" followed by "If so, can you resolve this MX record to an A (host) record and resolve its IP address?" When the client receives an answer from the server, it reads and interprets the MX record and gets the A record, thereby resolving the computer's IP address.
1. At a command prompt, type nslookup, and then press ENTER.
2. Type server <IP address>,where IP address is the IP address of your external DNS server.
3. Type set q=MX, and then press ENTER.
4. Type <domain name>, where domain name is the name of your domain, and then press ENTER. The MX record for the domain you entered should be displayed. If the MX record is not displayed, DNS is not configured properly.
The example below shows how MX records appear for the fictitious domain, example.com.
C:\> nslookup
Default Server: pdc.edu.pk
Address: 192.168.3.10
> server 111.68.97.17
Default Server: dns1.example.com
Address: 111.68.97.17
> set q=mx
> pdc.edu.pk.
Server: dns1.pdc.edu.pk
Address: 111.68.97.17
pdc.edu.pk MX preference = 10, mail exchanger = mail1.pdc.edu.pk
pdc.edu.pk MX preference = 10, mail exchanger = mail2.pdc.edu.pk
pdc.edu.pk MX preference = 10, mail exchanger = mail3.pdc.edu.pk
pdc.edu.pk MX preference = 10, mail exchanger = mail4.pdc.edu.pk
pdc.edu.pk MX preference = 10, mail exchanger = mail5.pdc.edu.pk
mail1.pdc.edu.pk internet address = 111.68.99.10
mail2.pdc.edu.pk internet address = 111.68.99.11
mail3.pdc.edu.pk internet address = 111.68.99.12
mail4.pdc.edu.pk internet address = 111.68.99.13
mail5.pdc.edu.pk internet address = 111.68.99.14
In this example, the preconfigured DNS server is behind a proxy server. Therefore, an external or Internet DNS server with a known IP address of 111.68.97.17 was used to perform the query. Next, the query type was set to MX to locate the mail exchangers for example.com. In this example, five SMTP servers are equally balanced, each with its own IP address. However, your domain might only have a single entry, as seen in the following example:
contoso.com MX preference = 10, mail exchanger = mailbox.pdc.com
1. mailbox.contoso.com internet address = 10.57.22.3
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