What are email headers?

All emails contain a standard set of information: The "To:" address, the "From:" address, the "Subject" and the "Date", all of which are easily viewable in all email clients. As well as this, information is put at the top of each email by the mail servers the message travels through - hence the term "headers". These headers can contain a lot of technical information, which makes them unnecessary for normal viewing, but can be very useful if there is a problem that needs to be investigated.