Re: you could do that, but...
"The only employees who are likely to need outside email access in most companies are the sales team"
Sales and marketing are the worst offenders in what they send from their businesses. They are the most addicted to sending HTML mail, the worst for embedding links and apt to use outside agencies so that the actual domain from which mail is sent isn't their own and the embedded links are also likely to belong to a different domain. In short their emails look exactly like phishing emails.
They expect other people to open their emails so why wouldn't they open those with exactly the same characteristics?