The 'Return-Path' is contained in the envelope header so it is generally not visible when viewing an email. In this case, the originator is a mailing list whose domain name (listname@domain.tld) is 'mailinglist@example.org'.

This field is important because it contains the fully qualified domain name of the originating sender's email account and cannot be forged. When an email is transferred from the originating MTA to the first receiving MTA, the receiving MTA checks to see whether the hostname that has been provided to it by the originating MTA resolves to a unique Internet address. If it does, then it is a fully qualified domain name and the receiving MTA adds the hostname to the 'Return-Path'. If the hostname does not resolve properly, the receiving MTA adds the originating MTA's IP address instead. Even if the 'From:' and 'Reply-to:' fields do not reveal the address of the originator, this information is available in the 'Return-Path' in the message's envelope header.

The 'Delivered-To' field is contained in the envelope headers so it is generally not visible when viewing an email. This header shows this email was delivered to the mailbox of a subscriber whose email address is 'subscriber@example.com'.

Since email messages generated from mailing lists are addressed to the listname alias, rather than individual subscribers, the 'To:' field in the message header doesn't match the value in the 'Delivered-To' field in the envelope header— so the 'To:' field would contain something like 'listname@example.org' and the 'Delivered-To' field would contain something like bob@marketing.example.com.

A 'Received' field is added to the envelope header for each step of the host-to-host delivery process. The MTA that receives the email adds a 'Received' field with information about the transfer, including the address of the sending MTA, the date and time of the transfer (according to its own 24-hour clock), the length of time it took to process the transfer and the type of email application it used.

This is the message header 'From:' field you see when you view an email message. When you receive an email from another person, the 'From:' field generally contains the person's email address (and name or nickname). When an email is posted to a mailing list, the mailing list software sets the 'From:' field to the listname to protect the private email address of the individual who posted the message.

This is the message header 'To:' field that you see when you view email. In this example, it contains the list address because the message was generated by a mailing list, rather than an individual. If a friend or colleague sent you an email, this field would contain your email address.

This field is similar to the address you'd add to an envelope to send a message through the postal service.

This is the message header 'Subject:' field that you see when you view email.

If this field contains the phrase 'ezmlm warning', the mailing list has encountered a problem delivering the message and sent the sender a warning message. See When the subject is an ezmlm warning for more information.

If you are troubleshooting an email that appears to have vanished and this field is blank, you have found the source of the problem. Spammers frequently send messages with empty 'Subject:' fields, so spam filters check the 'Subject:' field to see if it's empty, and if it is, the message is classified as spam and deleted. Since it would be bad policy—as well as a waste of system resources—to provide a spammer with information about why their broadcast spam message failed, messages deleted by spam filters fail silently. For more information, see Virus Scanning and Spam Blocking.

This is the message header 'Date' field that you see when you view email. It is set by the originating host, so it is not necessarily synchronized with the clocks on other hosts it may have encountered en route.

This field is analogous to the return address on a message you'd send through the postal service. As with hardcopy mail, you must have the addressee's information correctly or the mail may be undeliverable, but you could write anything for the return address and the mail would still be deliverable (if not returnable).

If the message is from another person, their email address will usually appear in this field. If this message is from a mailing list, the 'Reply-To' field will contain the alias for an automated bounce-handler, so that bounced messages are handled properly, rather than being posted to the list and forwarded to all the list subscribers.