When is it more professional not to send a business email

Disagreements

If you are having a disagreement with a coworker, or see potential for one, career coach Farnoosh Brock recommends you take the conversation offline. It's easier to fly off the handle over a keyboard and say things you'd never say in person, plus "the last thing you need is a miscommunication via email to make matters worse."

when the content is highly sensitive or emotional