Reaction notifications
The first time someone reacts to one of your posts, you get a private message from them saying how they reacted, with a link to the post.

You hear about each person once per post. If they change their reaction, or take it back and react again, you don’t get another message. You’re never told about your own reactions.
Turn reaction messages off
Section titled “Turn reaction messages off”-
Go to User CP, then Edit Options.
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Under the private messaging options, untick Send me a private message when someone reacts to my posts, and save.

You also won’t get them if you’ve turned private messages off, or put the person on your ignore list.