Have you ever wondered how your emails get to the right inbox? What helps the email server sort and deliver them on time? Are there address details like a house number and street name in an email, like there is on a postcard? Yes, each email will have an address in the form of an email header.
The email header enables proper sorting and timely delivery. The header contains details that enable email servers to identify the sender and receiver, ensuring the email reaches the intended recipient's inbox.
Email header
The header field of an email contains the address information. The RFC standard defines the structure of email headers, and adhering to this standard is crucial for ensuring successful email delivery.
A typical email header includes a field name followed by a colon (:), and then the field body with the relevant information. Important fields in an email header include the email origination date, sender address, receiver address, return path, and message ID.
According to RFC 5322, some email header fields are mandatory while others are optional. The email header can contain multiple instances of a specific field, but certain fields are limited to only one occurrence.
Mandatory fields that are limited to a single occurrence
Date: The "Date:" field shows when the email entered the delivery system. The date and time the user clicks "Send" is considered the email's origination date and time.
Even when the user isn’t connected to a network, the time at which the "Send" button is clicked will be considered as the email origination date and time. The email will be queued and sent to the recipients once the user connects to a network.
From: The "From:" field contains the email address of the user from whose mailbox the email is being sent. For example, if a secretary sends an email using an email address delegated to them, the "From:" field will list the original owner's email address instead of the secretary's mailbox address.
Optional fields that are limited to a single occurrence
Sender: The "Sender:" field shows the email address of the person who actually performs the sending action. For example, if a user's email address is delegated to their secretary, and the secretary sends the emails on behalf of the original owner using the owner's email address, the "Sender:" field will have the email address of the secretary, and the owner's email is put in the "From:" field.
If the "Sender:" and the "From:" email addresses are the same, the "Sender:" field will be left out of the email header.
Reply-To: The "Reply-To:" field specifies the email address to which a reply should be sent. This address is designated by the sender of the original email. If the "Reply-To:" field isn’t present in the email header, the email address in the "From:" field is used by default for replies.
However, the user composing the reply can choose to change this address.
To: The "To:" field contains the email addresses of the primary recipient(s) of the email.
Cc: Cc denotes carbon copy, referring to the now-antiquated practice of making a copy while typing or writing using carbon paper. The "Cc:" field specifies the email addresses of recipients who should receive a copy of the email, although the content is not directly addressed to them.
Bcc: Bcc stands for blind carbon copy, and the "Bcc:" field specifies the email addresses of recipients who shouldn’t be revealed to other recipients of the email.
Subject: The "Subject:" field specifies the context of the email content to the recipients.
Message-ID: The "Message-ID:" field specifies the email's unique identifier. This is typically a string of characters enclosed in angle brackets, "<" and ">".
In-Reply-To: The "In-Reply-To:" field is added when an email is being replied to. This field will have the contents of the "Message-ID:" field of the email to which this email is being sent as a reply.
References: References are added to an email conversation. An email conversation will have multiple messages and hence have the "References:", "In-Reply-To:", or "Message-ID:" fields.
This field will contain the content of the parent email's "References:" field, followed by the content of the "In-Reply-To:" and "Message-ID:" fields.
Email headers matter
Administrators of an organization use email headers to check the configuration details of their email service and ensure the successful delivery of emails sent from their organization. This also helps them analyze the routing of the email traffic into their organization, keep spam emails in check, and ensure a healthy inbox for their users.
As recipients, users can analyze the sender and recipient details along with other header field values to identify and act upon phishing and spoofing emails.
Zoho Mail's compliance to RFC 5322 header standards
With the email header in place and governed by the RFC standards, email service providers adhere to these guidelines to provide their users with a safe and secure email experience. We would like to bring to your attention that any emails received by Zoho Mail that do not comply with the header standards as per RFC 5322 will be rejected.
This policy will take effect on May 31, 2025. We recommend that you review the emails received by your organization and ensure that the headers comply with the standards to maintain uninterrupted incoming traffic for your organization's users.
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