Policies - Custom Headers

This article contains information on configuring custom Email Headers in Mimecast, including steps to create and manage policies, define header names and values, and set conditions for applying headers to specific senders, recipients, or IP ranges.

Also see Custom Header Feature - FAQ.

Custom Header Feature Overview

The Custom Header feature provides you with the ability to personalize your organization's email headers. This is achieved through the creation of a policy that allows the addition of an Email Header with a value of the administrator’s choosing. The application of the Custom Header is based on certain conditions you need to define. 

You can enable the Custom Header, by using the following steps:

1. Log in to the Mimecast Administration Console.
2. Navigate to Policies | Custom Header Policies.
3. Click on Create New Policy.
4. Enter the details for the policy:

Field / Option	Required configuration
Name	Enter a name for the policy.
Policy Description	Enter a description for the policy (Optional).
Custom Header name	Enter the custom header’s name, e.g. “X-Type.”  You can add up to 128 characters The following words are reserved, and not allowed in custom header names: "Mimecast", "Google", "Microsoft", "MC", "MS", and "OriginatorOrg", also Message Header terms, e.g. “To,” “From,” “Content-Description”.  Section 3.6 from RFC 5322 outlines the list of reserved headers. You do not need to add the “:” at the end. Non-ASCII can be used via UTF-8 encoding, as outlined in RFC 2047.
Custom Header Value	Enter the custom header value (Optional). E.g. “Application,” or “Invoice.”  The Custom Header Value can be any combination of alphanumeric characters, including 3 spaces. Non-ASCII can be used via UTF-8 encoding, as outlined in RFC 2047. You can add up to 256 characters. Variables are not supported.
Policy status	Activate or deactivate the policy.
Ignore signed messages	The policy won't affect signed messages.
Bi-Directional	If selected, the policy applies to both incoming and outgoing emails.
Allow Policy Override	If selected, this policy will be considered first, rather than being considered in the normal priority, which is most-specific scoping to least-specific scoping.  Only one Custom Header Policy will be applied to a given message. For more information on policy specificity, see Mimecast Policy Specificity.
From	Specify the sender characteristics the policy is based on. The options are: Everyone: Includes all internal and external users. Domain: This enables you to specify a domain name to which this policy is applied. Email Address: This enables you to specify an SMTP address.  Address Groups: This option enables you to specify a directory or local group. Once it has been enabled, a Group field appears. Click the Select Group button to choose a group from the directory group list.
To	Specify the recipient characteristics the policy is based on. The options are: Everyone: Includes all internal and external users. Domain: This enables you to specify a domain name to which this policy is applied. Email Address: This enables you to specify an SMTP address. Address Groups: This option enables you to specify a directory or local group. Once it has been enabled, a Group field appears. Click the Select Group button to choose a group from the directory group list.
Hostname or IP	Administrators have the option to specify a hostname or public IP range. The conditions set in the policy will only apply when the email is sent from the specified subnet (Optional).
Add Another Rule	Specify additional sets of conditions for both the "From" and "To" fields in the policy (Optional).

5. Click Save.