What to do if your transactional emails get spammed

None of the site owners would like their transactional emails * to fall into the spam mailboxes of users.

And users do not need this either. Imagine that you are waiting for a notice of receipt of goods on sale, and this letter has been in your spam box for a week already.

I thought of such a decision.
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1. The form that the user fills in on the site contains a field for entering “anti-spam words” (the user once invented it himself and uses it everywhere).

2. Any email sent by the server contains in the header (or in the body) this anti-spam word.

3. A user in his email client once sets up a filter that does not allow emails containing an anti-spam word to get into the spam box.

Antispam words in the general case can be any number, but it's easier, of course, to have one.

What are the pitfalls here?

* transactional email - usually this letter, which is usually sent after some user action, for example, confirmation of registration, password recovery, order in the online store, notification of receipt of goods and so on. etc.