[upon_drumhead]

About 5 years ago, my CEO emailed me with something like 'yo! can you do me a favor'. Middle management was CCed on the email thread and when I responded something like 'Sure! Happy to help', I was chewed out by my director for being informal and thus disrespectful...

[jyunwai]

That is an excessive response by the director. I wonder if the CEO would have approved of the director's comment if you emailed an apology to the CEO, identifying that the director made the comment ("I apologize for the excessive informal language in my email, as noted by Director X; I look forward to completing the assignment."), as the CEO may have deliberately been trying to create an environment where seniors are more approachable.

In any case, I try to always write in full sentences for professional communications as a minimum (even if there are sprinkles of informal language)—I'm more comfortable writing this way at work, and it has the added benefit that it's hard for a third party to make a similar comment like you've experienced.

(At the other end, it can also be seen as too stiff and uncomfortable to be overly formal, even to supervisors. I've seen this where I had a direct report who was excessively formal at first. By encouraging him to open up and speak more openly, I believe that created a foundation for a more trusting work environment over time, which encouraged him to take more initiative and share his ideas when solving some problems that we were later facing as a team.)