At times we get an error warning by our email client e.g. GMail in this case for an email that was recieved.
This message may not be sent by....phishing scam?
What is Phishing?
Basically what is telling us that someone may have sent you this email acting/impersonating as that person but the person who owns that email address did not actually sent it, indicating this is a spam or a possible fraud. So please be carefully while opening or clicking on any link in the email like this and when in doubt you can confirm back with the person to find out if in fact they sent it and if they did not , then report this as a phishing scam with your email provider.