In the business world, some information is meant to be seen only by a particular recipient. If you need to send a letter but do not want a secretary or other gatekeeper reading its contents, you will have to address it accordingly. Marking an envelope as private is simple.
Use a confidential or security envelope. Find these at any drug or office supply store. These letter-sized enclosures are printed with a design on the inside that prevents view of the contents they hold, even when held up to light.
Address your envelope. For a postal letter, write or type and print your name and address in the upper left hand corner. Write or type and print the recipient's name and address in the centre. Affix your stamp in the upper right hand corner.
Mark the envelope as private. For a postal letter, write or type and print "CONFIDENTIAL" or "PRIVATE" on the bottom of the envelope in upper case letters. Alternatively, stamp the word "confidential" with a rubber stamp and red ink in the same place.
If you are sending a private document through interoffice mail, first enclose it using Steps 1 through 3 above. Then, insert it into a manila interoffice envelope printed with the word "Confidential" at the top. Write the recipient's name on the next available space on the envelope. Include their department and office number if necessary. Finally, stamp "confidential" over the recipient's name, being careful not to obscure the delivery information.