If you don't want to waste time (and money):
1. You suspect your written message may be misunderstood or misconstrued.
Emotional written messages are likely to be interpreted according to the mood of the recipient and not by the intention of the sender.
2. You want to deliver bad news or discuss an emotionally charged matter.
Without the benefit of facial expressions, intonation, and body language, hurt feelings could ensue and flame wars could erupt if you deliver bad news electronically.
3. You hold a give-and-take conversation or want to conduct negotiations.
Dialogues that call for back-and-forth discussion are best be held on the phone or in person. Examples: To negotiate a price reduction with a supplier or persuade your supervisor to give you a pay raise.
4. You want an immediate or urgent answers.
Remember, unless a prior agreement, your recipients are not there, waiting for your emails.
5. A message is extremely important or confidential, and you cannot risk a breach of privacy.
E-mail is not a secure medium. So, never use this channel to communicate sensitive corporate information. There are millions of hidden readers and dastardly hackers lurking in cyberspace.
More suggestions on how to save time and money in my book Email at the workplace.