If the confirmation email generated by one of your Soapbox Engage app forms has a strange character in it that looks like a question mark in a black diamond then you likely have a hidden character that some browsers are unable to render or ignore.
If you open the form's Messages tab, and click the "Show/Hide" link of the offending field, you will be able to see the HTML of the email. Look for the spot in the text there where you saw the symbol in the email. You will likely see a little bit of mysterious code there. It may look something like this:
Delete that, and your symbol should go away.
Alternatively, you may have a "special" character that the browser can't render. It may be a curly apostrophe instead of a straight one, or a vowel with an accent, or an em-dash. Just delete that character and replace it with one type by your keyboard.
Why does this happen? It often happens when you copy and paste from elsewhere, and the illegal characters are carried over. Sometimes the WYSIWYG may also introduce invisible characters.