This is an example fetched from RFC 2822 : Internet Message Format to get a glimpse of the format of e-mails
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Mail Conversation Headers
Have a close look at Message-ID and References fields also...
Scene 1 : John Sends a Mail to Mary
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From: John Doe 
To: Mary Smith 
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>
This is a message just to say hello.
So, "Hello".
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When sending replies, the Subject field is often retained, though prepended with "Re: "
Scene 2 : Mary Sends a Reply Mail to John, (Note : She has a different Reply-To Address)
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From: Mary Smith 
To: John Doe 
Reply-To: "Mary Smith: Personal Account" 
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 10:01:10 -0600
Message-ID: <3456@example.net>
In-Reply-To: <1234@local.machine.example>
References: <1234@local.machine.example>
This is a reply to your hello.
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   Note the "Reply-To:" field in the above message.  When John replies
   to Mary's message above, the reply should go to the address in the
   "Reply-To:" field instead of the address in the "From:" field.
Scene 2 : John Sends a Reply Mail to Mary @ her Reply-To Address
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To: "Mary Smith: Personal Account" 
From: John Doe 
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 11:00:00 -0600
Message-ID: 
In-Reply-To: <3456@example.net>
References: <1234@local.machine.example> <3456@example.net>
This is a reply to your reply.
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