Just to give you an idea of how the heuristics protection works – any exe or other executable attachment will pop up that “Suspicious” warning even if the file itself is totally clean, simply because it’s a suspicious type of attachment.
And of course you should know not to touch exe attachments unless they’re from a source you know and you’ve specifically requested them. That’s almost the number-one rule of basic computer security even (especially?) for newbies. And of course even if they were something you wanted and were expecting, scan them before you do anything else with them.