Letters, whether handwritten or typewritten, take an investment of time to write, and then there is the long wait, usually several days at best, for them to be delivered and for a response to get back to you. With e-mail, the whole process can sometimes be completed in a couple of minutes.
Letters also require a certain formality of structure: a date, a greeting of "Dear So-and So," even when writing to a complete stranger, and a closing of "Sincerely," "Yours truly," "Love" or whatever. Then there is the signature. Should you just sign your first name or your entire name? Email frees you from this formality. You simply say what you have to say. The email system itself usually supplies the date and notifies the recipient of the sender. For busy people, email was a godsend. And then came texting, which is even faster and less formal.
And yet maybe email (not to mention texting and tweeting, right President Trump?) is just too easy. Fast messages can get us in trouble, especially if we are angry and hit the send button before we have had a chance to cool down and think over our message rationally. It is also easy to make mistakes. Meanwhile, the lack of formality can itself be offensive, especially when the recipient is someone who expects a little respect, such as an older person or a powerful person in government or business.
"To close the email," Cross wrote, "you can't go wrong with 'Sincerely,' 'Best' or 'Kind regards ..."
In other words, in business, especially at the onset of a new relationship, email should be more like old-fashioned letters, more formal and with a little more time taken to get it right.