If the word is spelled correctly, and the spell checker doesn’t know it, users can just add it to the personal dictionary, so you won’t see the error anymore.
I’m aware that you can add a word to the dictionary, that’s not the issue.
The irritating thing is that any unknown word is marked as misspelled, which is simply incorrect, there’s just a chance that it may be incorrect, but it’s definitely true that it’s unknown to the spell checker. Therefore I want to suggest changing the text to “unknown word”.
“Walt” may be unknown to the spell checker, but that doesn’t make it misspelled. And depending on how often I exchange email with someone named Walt I may opt to add it to my dictionary by way of the clearly available option.