How to export Outlook email to .eml or .mbox format

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 pathe-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.

How to View a Windows Mail .EML File?

So you have a Windows Mail .eml file?  What is it, and how can you make sense of it?

 It is quite common to encounter .eml files as email attachments.  To understand them, let’s consider how a .eml file may come to exist.  If you are a Windows Live Mail user, you may find that the process to save a e-mail message as a .eml file is quite simple. Knowing how this is done makes it easy to share messages with someone without having to forward the message. Sometimes the context of the email message is best preserved when the message is shared in its entirety as a separate file. EML files can simply be included in an email message as a file attachments, while a forwarded message is more susceptible to manipulation.
To create a .eml file using Windows Live Mail, start by going to the program’s main menu.   Second, single click on any email displayed in the message list (don’t open it, just highlight it).  Third, click the the far left button in the tool ribbon, choose “Save as File,” and the select a location to save the file.  Your message will be saved as a .eml file.
Windows live mail screen shot.
Saving a Windows Mail message as .eml (1)
Saving an email in Windows Live Mail as a .eml file.
In Windows Mail, save the message as a .eml file
Another way to create a .eml file from Windows Live Mail is to just drag it out of the inbox and drop it on your desktop. That works, too.

What is a .EML file?

If you want to get technical about it, a  file with a .eml extension should conform to RFC-2822, the internet message format.  This standard establishes a common syntax for text messages so they can be sent between computer users as e-mail. Each .eml file contains a single email message (unless of course another .eml file is embedded inside of the .eml file, but each .eml file is really a separate instance of an email message).  
Most .EML files you will encounter are created by popular email clients such as Microsoft Outlook Express, Windows Live Mail, or Mozilla Thunderbird.  To create a .eml file using either of those two email clients, all you have to do is drag an email message from the inbox and drop it on your desktop.  Voila! Instant .eml file which will contain a header in plain ASCII text,  the main body of the message (text or html), and any file attachments associated with the email, such as spreadsheets, jpg pictures, videos, or text documents.
.EML files can range in size from 1 kilobyte to many megabytes, depending of course on what the sender has chosen to put into the file.
The truth is, not all .eml files conform exactly to RFC-2822. In the wild, you are likely to encounter many exceptions. Different software products that generate .eml files sometimes fail to implement the entire RFC standard. When this happens, emails may not be formatted correctly, or are unable to be opened.  PstViewer Pro is a .eml viewer software application that has identified dozens of these exceptions, and is therefore able to render many non-standard .eml files.
Often many email messages are grouped into larger files, such as a .PST file (Outlook) or a MBOX file (Thunderbird).  While Thunderbird and Windows Mail will allow you to save a single message as a .eml file, Microsoft Outlook does not allow this. Rather, Outlook saves individual emails as a .msg file, which is a Microsoft Office message format that is not RFC-2822 compatible.  As of Outlook 2010, Microsoft Outlook does not save email messages as .eml files.  MessageExport is a third party add-in for Microsoft Outlook that adds this ability to Outlook, allowing Outlook emails to be stored in EML format.