Viewing and Understanding Email Internet Transport Headers in .EML Files

A screen shot of Pst Viewer Pro displaying email internet header info.


An EML (Electronic Mail) file is an open file format used to store individual email messages. It is a standard file format often associated with email clients like Thunderbird that follow the MIME (Multipurpose Internet Mail Extensions) standard. EML files contain the complete content of an email message, including the text, attachments, and metadata.

An email internet transport header, also known as an email header or message header, is a block of metadata information that provides details about the origin, route, and content of an email message. This information is not typically visible in the body of the email when you read it but can be viewed by accessing the email’s header section.

Email internet headers are usually not visible to users. A useful tool for managing EML files and viewing the internet header is a Pst Viewer Pro (formerly known as EML Viewer Pro) by Encryptomatic LLC, a U.S. based software developer.

Here are some key components typically found in an email internet header:

  1. Return-Path:
  • The email address to which bounce notifications and delivery errors are sent.
  1. Received:
  • This section contains information about the servers through which the email passed on its way to the recipient. It includes details like the server’s IP address, timestamp, and authentication information.
  1. From:
  • The email address of the sender.
  1. To:
  • The email address of the recipient.
  1. Subject:
  • The subject line of the email.
  1. Date:
  • The timestamp indicating when the email was sent.
  1. Message-ID:
  • A unique identifier assigned to the email for tracking and reference purposes.
  1. MIME-Version:
  • The version of the Multipurpose Internet Mail Extensions (MIME) standard used in the email. MIME is a standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs.
  1. Content-Type:
  • Specifies the type of content contained in the email, such as text, HTML, or attachments.
  1. X-Headers:
    • Additional custom headers that may be added by email servers or applications for specific purposes.

The email header is crucial for various reasons, including:

  • Routing Information: It shows the servers and paths the email took to reach its destination.
  • Authentication: Some headers provide information on email authentication methods used, helping to verify the legitimacy of the sender.
  • Spam Filtering: Email headers can contain information used by spam filters to assess the likelihood that an email is spam.
  • Troubleshooting: When dealing with email delivery issues, the header can be analyzed to identify potential problems in the routing process.

To view the email header in most email clients, you can look for an option like “View Source,” “Show Original,” or “View Message Source.” Analyzing email headers can be useful in understanding the journey of an email and in troubleshooting email-related problems.

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How to Export Outlook Email to .eml or .mbox Format

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What are .PST and .MBOX files?

Pst files and Mbox files both email messages.  While PST files proprietary to Microsoft Outlook, Mbox is an open specification used by email clients such as Mozilla Thunderbird.  RFC-822 and subsequently RFC-2822 are the open specifications documents that describe internet messaging and embody the structure of .eml files.   An MBOX file is a “mailbox” file which contains a collection of RFC-2822 email.  So .MSG is to .EML what .PST is to .MBOX.

RFC-822 was not directly adopted by Microsoft Outlook, the leading email client software in the world. Outlook email messages are stored in a single .PST file which may contain thousands of individual emails, messages including their file attachments. The specification for  creating a .pst file is proprietary, and is determined by Microsoft Corporation. The .pst specification was closed until Microsoft began publishing parts of the specification in 2010.
When Outlook exports a single email item from a .pst file, it is saved as a .msg file, which is also a proprietary Microsoft specification. Outlook .msg and .pst files requires Microsoft Outlook, or an Outlook viewer such as PST Viewer Pro.
Emails within a .mbox mailbox are concatenated and stored as in plain text. The start of a message within the .mbox file is delineated by a line with the word “From” followed by a space and the return path e-mail address.
Because RFC-822/2822 is an open specification it has been adopted by email client software such as Mozilla Thunderbird. It is still used to store and exchange email messages. Some companies prefer to store emails in this open standard, rather than in a proprietary standard.

Exporting Outlook to .EML/.MBOX

Microsoft Outlook does not directly support exporting email messages from a .pst or .msg files to .mbox or .eml files. To do this requires a third party add-in extension to Microsoft Outlook.
MessageExport is a useful add-in for Microsoft Outlook for exporting email messages to formats that missing from Outlook. MessageExport integrates tightly with Outlook’s main toolbar to allow users to select email messages from within Outlook and then export the emails directly to .eml or .mbox format.
Choosing the "Export to .eml" email export function.
Export to .EML from Outlook toolbar
In addition to .eml and .mbox, MessageExport also adds other export formats to Outlook, such as .pdf, .msg, mhtml, gif, tif, text, html, and many others.
Image shows dozens of email export operations that can be performed by MessageExport add-in for MS Outlook.
Export formats included with MessageExport
To export Outlook emails to PDF with MessageExport, click a message or messages in the Outlook message list so that they are highlighted. Next, select the export format from the MessageExport dropdown list (.eml or .mbox) and then click the Export button.  MessageExport may ask you to select a target folder, and once done will begin to export the selected emails.
If you export to .mbox, then all of your selected Outlook emails will be in a single .mbox file.
If you export to .eml, then you will have one .eml file for each Outlook email that was selected.
MessageExport also includes a “drag and drop” conversion capability.  Using MessageExport’s Export Folder Explorer, you can immediately convert a message to the selected format just by dragging that file and dropping on a folder in the explorer.
Drag and drop email conversion from MS Outlook.
Export folder explorer in closed state, resides on the right-hand side of the Outlook window. To use it, click to expand.
Selecting the appropriate folder for despositing email that has been converted.
Export folder explorer in expanded state.  Drag and drop email messages on the selected folder to immediately convert it to the currently selected format.
 MessageExport is a useful add-in for convert Outlook email message to .eml, .mbox, .pdf and other formats that are not natively included with Microsoft Outlook.  You can download a free 15 day trial of MessageExport.