E-Mail delivery mechanism

Question

Background

I am working with email headers for spam detection.Each E-Mail header contains many "Received-from" fields,each "Received From" field added by an intermediate Smtp/Mail server in the path from Origin Of E-Mail to Destination of E-Mail.

Thoughts

I have read that Email delivery is done by Sender's Mail Server sending E-Mail directly to Receiptent's Domain Mail-Server(obtained IP address of receiptent Mail-Server through MX Query of Receiptent Domain)

Query

Can somebody explain,if the above method is right,then where does these Intermediate Mail-Servers come into picture and how does our E-Mail reaches to these intermediate Mail-Servers in the Internet.

Answer 1

This statement is not accurate:

" Email delivery is done by Sender's Mail Server sending E-Mail directly to Receiptent's Domain Mail-Server"

For example, at my firm, when I send an email from my mail client, I might not have an externally-facing SMTP server at my site. Mail might be routed within my company's Exchange infrastructure. Then, it might pass through some application on a non-Exchange server to do message hygiene functions like AV, or content examination or even stripping. Then, it might get sent to an external mail sending company like Mimecast, which certainly has more than one SMTP server. Then, Mimecast will probably do an MX lookup for the recipient's mail handler, which might be an external-to-them mail service - this is getting fairly common these days. Once they pick it up, it could go through several other servers, in a similar fashion to the above.

Answer 2

99% of the time, email is delivered via the MX record mechanism. The MX record designates which server is authoritatively responsible for accepting email for a given domain. But that doesn't mean that this server is also the server where the recipients mailbox is. The MX record only designates which server to deliver the email to, it doesn't designate which server hosts the recipients mailbox. So there may be any number of servers that handle the email once it's been accepted by the server that is designated in the MX record.

Additionally, the server that the outbound email client sends email to may not be the server that ultimately sends the email to the recipient. There may be any number of servers involved in relaying the email to the recipient.

So you see, from both the sending and receiving sides, there may be any number of servers involved in getting the email from the sender to the recipient.