Before understanding how Simple Mail Transfer Protocol (SMTP) Smart Host operates, it is important to understand the fundamentals of SMTP Direct Mode.
When Strategy Narrowcast Server is transmitting in Direct mode, it attempts to connect directly to the recipient's e-mail server over the Internet. It locates this server through a multi-step process:
- Retrieve domain mail server records. Strategy Narrowcast Server isolates the Internet domain from a recipient's e-mail address (for example, Strategy.com) by taking the characters following the '@' symbol. It, then, makes a request to the Domain Name Server (DNS) server configured for the available network interface cards, asking for the 'MX' or mail exchanger record for the domain.
- Locate mail server. Strategy Narrowcast Server isolates the name of the mail server with the highest priority from the domain mail exchanger record. It, then, determines the IP address for that mail server using a combination of DNS and WINS requests.
- Open a TCP/IP connection. Strategy Narrowcast Server attempts to open a TCP/IP connection on the port specified in the E-mail (SMTP) information transmitter settings, to the remote mail server. If a connection cannot be established, Strategy Narrowcast Server tries to locate another of the e-mail servers returned in the domain mail exchanger record.
- Open a mail session. If a connection was successful, Strategy Narrowcast Server opens up an e-mail session using EHLO (E-SMTP) if possible. Otherwise, HELO (SMTP) is attempted. If both protocols are rejected, Strategy Narrowcast Server returns to Step 2 and tries to locate another of the e-mail servers returned in the domain mail exchanger record.
- Send a message. Depending on the session type, Strategy Narrowcast Server uses the SMTP or E-SMTP protocol to deliver the e-mail message to the remote e-mail server.
- Locate the smart host. If all e-mail servers listed in the domain mail exchanger record are unreachable or unworkable, Strategy Narrowcast Server attempts to deliver the message using the smart host if it has been configured. It locates this mail server using a similar process to Step 2, and proceeds with steps 3-5. If this process fails, the message is returned to the pending delivery queue.
When Strategy Narrowcast Server is transmitting in Smart host mode, it only attempts to connect to the e-mail server configured in the device settings. It proceeds through steps 3-5 in direct mode. If the process fails, the message is returned to the pending delivery queue.
A smart host is a designated server through which a mail server routes all outgoing messages. The smart host then makes the remote connection. If a smart host is designated, the mail server only needs to transmit to the smart host, instead of repeatedly contacting the domain until a connection is made. Also known as a relay host. This feature is useful when a powerful e-mail server is used for all outgoing messages, since the communication between the Strategy Narrowcast Server and the smart host is faster than that between the Strategy Narrowcast Server and the mail host of each recipient.
The use of a smart host can help alleviate connectivity issues caused by some firewalls. For example, if the users' Strategy Narrowcast Server system must transmit to recipients outside of the corporate firewall, users can configure it to transmit to a smart host inside the firewall that already has Internet access, and then let the smart host perform the transmission to the recipients. Smart host configuration is part of the device configuration process.
