what causes some email to have the character ? inserted as a space?

when typing the email message from my gmail account using a Google Chrome browser I needed to create some simple columns of information and I used multiple spaces between each column’s data. This email was then sent to a large group of people with different types of email. Some of them report that my email message was had to read because it had the character ? (a with an accent mark) inserted as the spaces between each column. But the normal space between words was not affected.
What causes this and how does one prevent or correct it?