Notes on Using #classified/#external Keywords for Sending Confidential Email

  • Keyword #classified

    Keyword label #classified (case insensitive) in the email Subject field is for sending confidential email to internal recipients.

    Email with #classified in the email Subject field is ONLY allowed to be sent to internal recipients with @cityu.edu.hk domain (staff email domain). If it is sent to recipients outside staff email domain, including @staff.cityu.edu.hk and @my.cityu.edu.hk, and non CityU domain addresses, the email will bounce back to the sender with an un-deliverable message.

    For #classified email mixed with recipients of staff email and non-staff email domain, ONLY recipients of staff email domain will receive it.

  • Keyword #external

    Keyword label #external (case insensitive) in the email Subject field is for sending confidential email to recipients outside staff email domain.

    Please note that if email with #external in the email Subject field is sent to recipients with @cityu.edu.hk or @um.cityu.edu.hk domain (staff email domain), they will receive it without encryption.


Note: Keyword #classified and #external use different approaches to encrypt email. #external email use Office 365 Message Encryption (OME) technology. Compared to OME, #classified email use Rights Management Services (RMS) encryption, which is with higher level security. With RMS, MS Office documents attached to #classified email are natively protected with restricted permission even if they are downloaded/detached from the email.

The following table shows the behaviors on using the keywords for confidential email:

Keyword Sender Recipient Behavior
#classified @cityu.edu.hk (Office 365) @cityu.edu.hk (Office 365) RMS encryption applied
#classified @cityu.edu.hk (Office 365) @staff, @my Bounced back
#classified @cityu.edu.hk (Office 365) @gmail.com Bounced back
#external @cityu.edu.hk (Office 365) @cityu.edu.hk (Office 365) No encryption applied
#external @cityu.edu.hk (Office 365) @staff, @my OME encryption applied
#external @cityu.edu.hk (Office 365) @gmail.com OME encryption applied
csc@cityu.edu.hk