How to fix transactional (order) emails sending in Magento 2.2.5?

Issues
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1

Replace the line:
https://github.com/amzn/amazon-payments-magento-2-plugin/blob/v2.0.14/src/Payment/etc/di.xml#L63-L63

with the following one:

<type name="Magento\Framework\Mail\MessageInterface" />

Then re-compile the Magento code.

I have found the solution here:

2

The issue is fixed in the 2.0.15 version of the amzn/amazon-payments-magento-2-plugin package (and in Magento 2.2.6):