What is best practice to prevent man in the middle compromising emails?

Our office recently had a very clever email attack which was caught before any damages. Ive been directed to research possible future prevention of the same thing happening because Im pretty sure its a vulnerability outside of our organization and will start to be more common. An email communication was intercepted with an attachment that had our companies ACH info for the recipient end customer to provide a payment to. The WORD document was modified and then reforwarded to the …

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