Bloomberg – Matt Levine (read free): “The rule, according to the US Securities and Exchange Commission, is that if you work at an investment bank or a brokerage firm or an investment adviser, you are allowed to communicate with your colleagues and clients about business on “official channels,” and you are not allowed to communicate with them about business on “unofficial channels.” There are some obvious cases:

• You are allowed to use your firm’s email system, and Bloomberg messaging, and other electronic communications channels that are approved and monitored and archived by your firm.
• I guess you are allowed to put a letter in the mail? If you type it in Word on your office computer? Can’t imagine it comes up that often.
• You are not allowed to message your colleagues or clients about deals by WhatsApp or text message on your personal cell phone.

See also The New York Times – Banks Fined $549 Million Over Use of WhatsApp and Other Messaging Apps. “The penalties add to nearly $2 billion in record-keeping fines last year for the improper policing of employee communications on private messaging services.”