MailChimp has a very robust writing guide focused on voice and tone in multiple situations. I particularly like how they handle the customer’s emotion in negative situations:
User: What went wrong? I really need to get this campaign out.
User’s Feelings: Confusion, Stress, Anger
-Offer a solution or next step.
-Be straightforward. Explain what is going on right away.
-Be calm. Don’t use exclamation points or alarming words like “alert” or “immediately.”
-Be serious. Don’t joke around with people who are frustrated.
MailChimp: We’re experiencing a problem at one of our data centers. Our engineers are on the case, and will have things back to normal shortly.
MailChimp’s full voice and tone guide can be found here.