Hello,
We do not check "Do Not Send Marketing Materials" when sending emails, though we do check "Do Not Allow Bulk Email" or "Do Not Allow Email" and if either of those two are set to "Do Not Allow" then we won't send those people emails.
This particular customer you're looking at may have an Unsubscribe record or may have been Blocked due to too many bounces.
Thanks for using the forum and please let us know if you have questions!