I have edited the default "?" in the vocabulary editor only to make sure that it only has one space following. Dictation of "question mark" seems to work fine everywhere except MS Outlook. In Outlook emails, dictated "?" is in a smaller and different font. Here's a screenshot: https://i.imgur.com/zjKqBnr.png

My default font is Calibri 11px, and the "?" appears as MS Sans Serif 8px. Then of course every character after the "?" is in that smaller font.

What could possibly be causing this punctuation mark to change fonts? I'm just not able to find any troubleshooting that helps.

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Do you dictate a sentence, pause and then dictate a question-mark separately? If so, try dictating your entire sentence with the checkmark (without pausing).

We suspect you have accidentally created a Text & Graphics command which simply types ?. Try searching for this command and deleting it. Additionally, when creating Text & Graphics commands, which simply enter text, remember to add a checkmark to the lower right corner Plain Text box, which will also eliminate this problem.

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YES!!! This solved it!  

I had created a Text & Graphics command to type a "?", but totally forgotten about that. I had been having a lot of trouble with Dragon not recognizing me saying "question mark". It would type out "Russian mark" instead. I tried all kinds of things before finally thinking that I could create a new vocabulary item "Russian mark" and have Dragon type out "?".

Instead of deleting the Text & Graphics command, I simply followed your suggestion to tick the box for plain text. 

Thanks so much for sharing your expertise!