Nice idea, gmayor, and well explained, but it won't work. Here's the sequence:

1) I was typing in Outlook and started proofreading an email I'd just written. I noticed that Outlook did not red-underline "intsead", as it should. I supposed that I must accidentally have put "intsead" into Outlook's custom dictionary; I looked around in Outlook for that dictionary so I could delete "intsead" from it. All I could find was CUSTOM.DIC, and "intsead" wasn't in there.

2) In Firefox I came to this forum to ask where else I might find the offending dictionary. I carefully typed out "intsead" each time while posting the question, and each time Firefox red-underlined it—proving that Firefox and Outlook use a different dictionary, but then we would have assumed that.

3) Once I posted the message it was no longer subject to spell-checking, so if you look at my original 05-20-2016 04:05 PM post you'll see "intsead" is not red-underlined there.

4) You copied the text of my message into Outlook, and Outlook, using your spelling dictionary, red-underlined "intsead", even though mine did not.

The difference, I'm sure, is in the dictionary that contains the words that we tell spell-check to accept. When spellcheck red-underlines a word I've typed, it's because it doesn't recognize it. Usually that's because it's a proper noun or idiosyncratic abbreviation, such as "Thornqvist" or "odcdi", that the spell checker cannot be expected to know; in such cases I ignore the red underlining or, if I'm going to be using that word a lot over the next months or years, tell spellcheck to add that word to the dictionary.

For you, Outlook flags "intsead" as a misspelling, of course—it would for everyone. The fact that it doesn't for me leads me to believe that some time in the past I must have carelessly told spellcheck to add "intsead" to the dictionary, and I've only now noticed it. My question: Where to find that dictionary?