Possible Clues:

• Include secretly in an email, say
• Privately keep in the email loop, briefly
• Abbr. at the top of an email
• Secretly send a dupe email to
• Emailing option
• Include discreetly, on an email
• Covert communication
• Abbr. in an email field
• Discreet email initials
• Secretly 63-Across