Harrier, Isode's Email Messaging client, provides a modern user-interface to secure messaging on Web. Harrier can be used in both General Purpose and Military deployments, including as part of an ACP127 service.

Developed from the ground-up with the needs of secure messaging users in mind, Harrier provides an end user client service via a server component using SMTP messaging with RFC 6477 and related extensions to support military messaging (see the whitepaper [Military Messaging (MMHS) over SMTP] for more details). 

The Harrier Web Client is a JavaScript application that runs in the user's Web browser, and will be downloaded as and when needed. Harrier Web Client connects using HTTP (REST) and Web Sockets to Harrier Web Server, making use of JSON, CSS, and HTML.

The Harrier Web server server uses SMTP for message submission to M-Switch and IMAP for message access to M-Box. It supports Web access with modern browsers to provide high performance user friendly access.

General Purpose and Military Capabilities

Harrier can operate as a general purpose SMTP/IMAP client, providing a high performance easy to use Web interface to an IMAP/SMTP service. It provides a useful set of general purpose email capabilities and some features that are not generally available in standard email clients, including security label selection/display and time controls.

Additionally Harrier supports a wide range of Military Messaging capabilities, including:

  • Action and Information recipients, with associated priorities
  • Exempted Recipients
  • Security Label selection and display using RFC 7444 and flexible server configuration of security label choice
  • SIC (Subject Indicator Code) support with server side configuration of SIC catalogue
  • Message Type support with server side configuration of SIC catalogue
  • Time controls (Filing Time; DTG; Expires; Reply-By; Deliver-By)
  • Flexible Reply/Forwarding handling and display
  • Attachment handling and display
  • Handling Instructions
  • Use of Read Receipts to indicate message responsibility

Harrier supports TLS for all connections, and account setup defaults to use of TLS, so that not using TLS needs to be a specific user choice. Harrier verifies server digital certificates against client trust anchors. Where a certificate is not correctly trusted, the user will need to confirm certificate use prior to use of the connection.

Harrier for ACP127

Harrier can be used as an ACP127 client in conjunction with M-Switch. ACP127 can be mapped with SMTP and RFC 6477 / RFC 7444 without loss of ACP127 information. This enables Harrier to provide the full ACP127 service. In ACP127 mode:

  • Addresses are presented as ACP 127 RI (Routing Indicator) and PLA (Plain Language Address) with SMTP addressing hidden from the user.
  • Lines are limited to 69 characters
  • Character set is restricted to ITA2 or IA5
  • Attachments are disabled

Isode server products provide seamless gateway access to/from message systems using SMTP and X.400 as well as military protocols such as STANAG 4406, ACP127 and constrained bandwidth variations.

Key Features

Harrier supports a number of key features which make it ideal for use in secure environments including:

Security Labels

Support for Security Labels is based on RFC 7444 "Security Labels in Internet Email".  Isode's Security Label Server is used as the interface to enable Harrier to select a security label appropriate for the selected recipients. This client/server architecture, which allows flexible security labels to be provided without heavyweight client support, is described in detail in the whitepaper [Easy Security Label Support for Email Clients] .

Time-Related Control Capabilities

Harrier allows users to set time-related controls. The reply by, expires on and deliver by controls allow the prioritization of messages handling. This capability is useful for time critical messages being sent over less reliable networks.

SICs and Message Type

Subject Indicator Codes (SICs) and Message Type are widely used military messaging fields. Harrier can set and display these. The values used are stored in the Security Label Server, so values can be managed centrally and shared between clients.


Harrier is available now and can be evaluated alongside Isode's server products for Military Messaging. Please contact us using the Evaluation Request form for more information.