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Old 18 Feb 2017, 11:05 AM   #3
Intergalactic Postmaster
Join Date: May 2004
Location: Irving, Texas
Posts: 8,586
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Email started with a very simple system where you could actually log into a receiving mail server manually and type the message directly into the server. In order to allow troubleshooting and discover the source of early spam, the system evolved to the current system made up of several agents:
  • Email user client: The user enters the message into the email system using a client. The client then connects to a MTA (message transfer agent) and sends the message using SMTP protocol on a submittal port.
  • The MTA logs the IP address of the client and places this in the message headers. Early MTA's didn't need authentication (username and password) or a secure connection. So the IP address was important for maintaining a small degree of system security.
  • Each subsequent transfer of the message over the public internet also causes the IP address of the sending agent (MTA) to be logged into the headers.
  • Webmail changed this relationship. All users had to supply a username and password, and nearly all systems eventually required use of a secure connection. So now you weren't really creating the message locally on a client, but we're creating it using a web connection directly at the server. So the IP address which is logged into the headers is now the address of the MTA, not the end user.
  • The FastMail app for iOS or Android devices is just a modified version of the webmail system. It's supplied by FastMail and requires your master password so it's easier to trust.
  • Each message has a Message-Id header. This is a unique identifier which is different for each message, and among other uses allows email loops to be detected. The Message-Id is assigned by the submittal agent. In the case of an email client, the client generates this ID, while with webmail it is generated by the webmail server. So, in general, the first IP address you will see in the headers is the agent which generated the Message-Id.
  • No, you can't change this behavior. Use webmail or the FastMail iOS/Android app if you don't want the recipient to be able to track your IP. FastMail logs your IP internally, as they have to be able to discover the source if you do something against their terms of service, but they don't put your IP in the headers unless you use a conventional email client.
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