What's a sales situation where you should NEVER send an email?

Personally, I'd say to minimize email whenever possible in all communications. But to give you one example, don't send an email to follow up with a customer post-sale. It's disingenuous. The purpose of the follow-up is to get a read on whether the customer is finding value in what you've provided. You need to hear their voice or even better would be to visit them and read their body language. Any time you want to establish a real human-to-human conversation, avoid email if at all possible.

Here are a few...

1. When you need to tell prospect you cannot keep your promise.  Product, service or pricing no longer available.  Best to meet with them and share the "better opportunity".

2. Asking what problem you can solve for them, prospects are not great at putting in writing what their problems are.
3. Asking prospect what their budget is? Best to get them to talk first, but they will not want to put that in writing.

Pricing discussion should never be done via anything other than a meeting.