like: vertraulich → →
Diese Nachricht ist streng vertraulich! ☊ 
Bức thư / Văn bản trên đây tuyệt đối bảo mật. ☊ (Lektion 90, Nr. 1349) Diese E-Mail enthält vertrauliche Informationen. ☊ Bức thư này chứa đựng những thông tin bảo mật. ☊ (Lektion 91, Nr. 1376) access to confidential information ☊ access ☊
file on sb: to have/open/keep a confidential file on sb ☊ file ☊
I would be most grateful if you would keep this matter confidential. ☊ grateful ☊
Do not open any mail marked 'Confidential'. ☊ mark ☊
stress that...: I must stress that everything I've told you is strictly confidential. ☊ stress ☊
My letter is, of course, strictly private and confidential. ☊ strictly ☊