what does this smtp setting?

can someone tell me what the smtp setting Use domain name system [DNS] "MX"records to route mail automaically does?

im just curious how it works.

Joris DijkAsked:
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This is where you would setup how exchange will send messages

The first option (DNS) - Exchange will lookup the mail server for a domain (xyz.com) and attempt to deliver mail directly

The second option (Smart Host) - Exchange will send messages to be delivered "through" your ISP account
First Option DNS MX records: If you select that option emails will be directly routed to external domains validating the MX records of the domain. you just need to make sure that MX records are set properly for the domain.

Second Option Smart host: Smart host is basically a Anti Spam Gateway.  If you are using third party services to scan you emails for spams, you need to put the smart host name provided by the service provider from where you are going to take services, there are many service provider which provides these kind of services.

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Hasin Ahmed ChoudharyExchange AdministratorCommented:
First Option DNS MX records:  It will validate MX records for External domains.

If you have Split-DNS, make sure that DNS record is resolvable from your internal DNS servers.  
You can also use external DNS server lookup, by selecting that option. ( visible in screenshot provided by you)
DP230Network AdministratorCommented:
For example this diagram:

End users' PCs --------------- Switch ----------------- Mailbox Server ----------------------- Dell Sonicwall Antispam/Anivirus ------------- Internet

Your opt 1: route mail via Mailbox
Your opt 2 : route mail via Sonicwall (this is a smart-host)
Joris DijkAuthor Commented:
thanks for the answers
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