Possible Clues:

• Abbr. in an email
• Discreetly send a dupe email to
• Secretly loop in
• Secretly include on an email
• Covertly include on an email
• Include covertly, briefly
• Slyly include in an email, for short
• Discreet email inclusion
• Secretly keep in the email loop, briefly
• Discreet email option