What is the rule? Simply this: When writing an email, discuss only one topic. The corollary to the rule is that the subject line must accurately reflect the same topic.
After a career as US Navy Helicopter Test Pilot, Instructor, and Acquisition Professional, I fell in love with organizational and Personal Growth and Development, Leadership Training, and Communication/Public Speaking education. Now I inspire others to Dream more, Learn more, Do more, and Become more as leaders.