Most people know about the consequences of opening unknown mail but choose to ignore all the advice given on the subject. They soon find out what happens when they ignore the advice and then come running for help to put right their own wrongs.
I have a sister who does just that, so I have given up advising her on it, she accuses me of nagging her. Curiosity gets the better of most people, but it is important not to let it get the better of you.
I use MailWasher and delete all unknown and unwanted mail before I download it and have very few problems doing so.