Unfortunately, at the moment it is not possible to integrate Automatic Related Products module with the Follow Up Email extension.
However, for a scenario when a customer visits any product page or leaves the cart and receives a follow up email, a similar output can be achieved. To include more related products in the letter, the existing functionality of the Follow Up Email extension is enough. Modify the number of related items directly in the Follow Up Email letter templates. Please contact our support to get detailed instructions.