FYI, I have just seen email from an O 365 system with no "To:" header. "To" has not been a required header for a long time, but still I was not aware of any legitimate software that sends without it. Use of an empty list like "To: Undisclosed recipients:;" must be a lost art.
This might affect scoring of the MISSING_HEADERS rule eventually. (Despite the name it seems to mean only a missing "To" header.) -- Joseph Brennan Lead, Email and Systems Applications
