How email works

I use email everyday and i handle exhange server before. But im not 100% fully understand how email works. how it can detect whether the email is in whitelist or not. i know it detected from header but how.? can anyone can explain how email works in detail?
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It works as follows.
When you send an e-mail to let's say it will be send to your mailserver (being Exchange or a mailserver of your ISP). Your mailserver first checks if you are allowed to send e-mails externally to another domain then your own. If that is true it will do a DNS request on the internet for the MX (Mail Exchange) record of the domain you are sending the mail to (in this case and when it finds this record (which corresponds to an ip address) the mail is send that way.

There, the receiving server looks to see if the account someone actually exists. If so, the mail will be send to this users mailbox, if not you will get an NDR message back (Non Delivery Report). If user someone connects to his mailbox he will receive your message.

Is this what you wanted to know?
Mohammed TahirMicrosoft Exchange and O365 AdministratorCommented:
How Exchange 2013 Mail flow Works:

VirastaRUC Tech Consultant Commented:
Hi your question is very blunt.

Please be specific when you have question, when you trying to understand something.

What exactly you are trying to learn?

How Basic Mail flow works?
How Microsoft Exchange Mail flow works?
How Exchange 2003 Mail flow works? or Exchange 2007 Mail flow works? or Exchange 2010 Mail flow works?

You might be thinking there might not be much but there is.

So, here is possible options I can come up with.

Point out the area you want to understand.

Hope that helps :)
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tankergoblinAuthor Commented:
Hi rhandels, thank you for explanation. One question if the destination user only create a mx record with no A record for the mailing address. can the destination user receive the email?
Yes, because the MX record is affiliated with the e-mail that is being send. An A record is nothing more than a reference from a dns name (or host name) to an ip address.
Seth SimmonsSr. Systems AdministratorCommented:
if the destination user only create a mx record with no A record for the mailing address. can the destination user receive the email?

an MX record points to an existing A record
if has an MX record for but there is no A (hostname) record for mail delivery will fail

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Hey Seth,

Didn't now that. I thought the MX record was enough..
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