Gary got a spam email message and now it's been forwarded to everyone on his contact list. He got failure notices from the message being sent to addresses that didn't exist. Leo says that it's not unusual to get failure notices where he has no control. Spammers can "spoof" the return address by using another user's email address as the return address. Nothing he can do about that, but the good news is that they'll soon move on to another email.
Leo says it's more likely that malware has infected his system and by viewing the email, the virus has installed and gone into his email account to harvest his contacts. This is one of the dangers of having HTML turned on in an email account. He should go into his email settings and turn that off. Use good passwords to prevent brute force attacks from cracking the password. Also, he can tie his phone to his email account so that if someone tries to change the password, he'll get an alert on his phone and will have to input a code from the phone in order to change the password.