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What are 'headers' in email messages?

Headers are lines of data at the top of every email. They contain information about when it was sent, who sent it, to whom it was addressed and to whom it's been delivered, what was used to compose it, which servers it passed through and so on.

Normally, programs like Outlook, Outlook Web Application and Mac Mail will hide these headers, but they may be shown as diagnostic information if a message is rejected. Each line of a header usually starts with a word followed by a colon, for example:

From: (Joe Bloggs)
Subject: An example mail message
Date: 8 Mar 2013 22:46:07 GMT
Status: OR

When a message is rejected or 'bounced' it is usually returned to the sender with information in the headers to show what went wrong.

You can look at full message headers to give more information on the path the message has taken.

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This is question number 463, which appears in the following categories:

Created by Neale Type on 27 July 2001 and last updated by Gemma Sturtridge on 26 March 2020