Policies - Custom Headers - Frequently Asked Questions

Frequently Asked Questions

This article contains information on Mimecast's Custom Header feature, including FAQs about header naming conventions, value limits, reserved words, policy rules, and IP subnet configurations, as well as guidance on creating and managing custom header policies.

Q:	What value can I use for the Custom Header name?
A:	You can use any alphanumeric value without spaces for the custom header name, as long as it doesn’t conflict with reserved header fields, e.g. ‘Subject’, ‘From’, ‘To'.  The value should meet RFC 5322 guidelines. The maximum length is 128 characters.  Non-ASCII can be used via UTF-8 encoding, as outlined in RFC 2047.
Q:	What value can I specify for the Custom Header value?
A:	The custom header value can be a word, a phrase, or a combination of alphanumeric characters. Allowed input includes three spaces (because this is part of the RFC), however, we would advise you not to do this. The maximum length is 256 characters. Non-ASCII can be used via UTF-8 encoding, as outlined in RFC 2047.
Q:	Can I add a variable as a Custom Header value?
A:	Currently, adding a variable is not supported.
Q:	What is the length of the Policy Name?
A:	The policy name can be an alphanumeric value up to 80 characters in length.
Q:	What is the length of the Policy Description?
A:	The policy description is optional and can be up to 160 characters in length.
Q:	How many IP Subnets can I specify?
A:	You can specify up to 50 public IP subnets.
Q:	Can I add multiple rules in a single policy?
A:	Yes, you can define multiple rules in a single policy.
Q:	What are the reserved words in Custom Header Name?
A:	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.
Q:	Does the Custom Header policy add multiple headers?
A:	No, the custom header policy can only add a header to an email that matches a rule within the policy. The rule with the highest specificity takes priority, along with the most recently created policy.