Some spammers send fraudulent messages that try to trick you into sharing personal information like passwords or credit card numbers. This practice is called phishing.
What you need to know:
We strongly recommend that you avoid clicking links in these messages and do not reply to them. Spammers can send messages that appear to be from a person or company that you know, and might even hack into someone's email and send messages from that account. Therefore, please be wary of these messages, even if you know the sender. Please know that Google will never ask for your password or sensitive information over email.
Actions you can take:
If the message seems like an attempt to get your personal information, mark "Report phishing" for the message to help our system learn from such attempts.
If the message is clearly not malicious, you can click the "Not spam" button to move the message out of Spam and into your Inbox. You can also select "Report not phishing" to let us know that the message is legitimate (click the down arrow next to "Reply" to see this option). By marking the message as not spam or phishing, Gmail's system will learn from that example and be more accurate in marking messages in the future. There are also some things you can do to prevent legitimate email from being marked as spam.
Gmail treat sub-domains as spam