Understanding basic mail related terms: message header, reply separator, source code, message file

This article explains the differences between an internet message header, message reply separator, message source code, and message file.

What is an email message header?

Message headers (also known as email message internet headers or email headers) are a piece of information that is added to a message traveling through a mail server (including Exchange organizations). This information contains a message trace that allows you to check what servers the message went through (and in what order) as well as inspect additional attributes that were added to the message in the meantime. These additional attributes are added either by Microsoft Exchange itself or by some third-party software, and their usual purpose is to maintain the proper automated mail flow.

In Outlook, internet headers of a message can be accessed by opening an email in a new window and clicking File > Properties on the menu, or by clicking the expand (Outlook expand section/menu button) button located on the Message tab, in the lower-right corner of the Tags group (Fig. 1.).

Fig. 1. Accessing message headers in Outlook.

What is a reply separator?

An email reply separator is a part of an email that separates one message from another in an email conversation thread (Fig. 2.). A reply separator is added when you are replying to or forwarding an email. Each email client (including mobile apps) uses its very own reply separator.


Reply separators are used by CodeTwo software to recognize the right place to add a signature inside a conversation. If your signature is not added above the very last reply separator, see this Knowledge Base article.

Fig. 2. An email reply separator.


If reply separators in a conversation are missing, double-check that the signature does not contain empty paragraphs if the option to remove previous occurrences of a signature is enabled.

What is the source code of an email message?

The source code of an email is the content of this email encoded using the HTML syntax. The source code does not contain images or other essential information about the message (such as internet headers).

To view your message's source code in Outlook, right-click anywhere in the message and select View Source from the shortcut menu. Note that the HTML source code preview is available only for messages that were sent in the HTML format – it is not available in RTF and plain text messages.

What is a message file?

A message file is a legitimate copy of the message. Message files contain relevant data such as message headers, the source code along with attachments, and other properties (the message sender, recipient, date, and so on). Message files can be exported from a mailbox, and the standardized export formats are MSG or EML. To export an email from Outlook to an MSG message file, drag it to the desktop or any other folder (you can also use the File > Save As option on the menu to have access to more export formats).
See guidelines on how to export emails to MSG/EML

Email troubleshooting

When it comes to troubleshooting emails, it is better to provide the entire message file than only the message's source code, because the message file includes a lot more information (see details above).