An exception covers one specific purpose. It does not stretch to sales analytics: a pixel that is needed for one purpose does not become needed for reporting who opened an offer.
The French exceptions should not be carried over automatically to other countries. In Spain, the AEPD’s report 0011/2014 calls for active consent before the first advertising email with a pixel.
This answer names only the exceptions described in the documents HawkOpen relies on. HawkOpen does not decide whether an exception applies to your emails; it adds a notice with a link to turn tracking off to every tracked email.
Where rules that require consent apply, tracking may start only after that consent has been obtained. The link in the message lets the recipient withdraw consent later.