Marking the messages as spam may result in legitimate emails from your contacts getting filtered in the future (because it is coming from their email address). I would recommend creating an email filter to stop these messages from spreading. Here are instructions for Gmail:
- Open Gmail.
- Click the gear in the top right.
- Select Settings.
- Click the Filters tab.
- Click “Create a new filter” near the bottom centre of the page.
- Enter (without quotes) “*If you cannot open it, move this message to the “Inbox” folder.” in “Has the words”
- Click “Create filter with this search”
- Check “Delete it” and click “Create filter”