Did they mark it as spam or as suspicious? OP said it was marked as suspicious. Spam and suspicious are not the same thing. It is entirely understandable that

Did they mark it as spam or as suspicious? OP said it was marked as suspicious. Spam and suspicious are not the same thing. It is entirely understandable that they might have marked something as suspicious. Compromised email accounts from known senders are a super popular way to scam people.