Click on the email addres full header info instead of the abbreviated header details (typical default).
1. You may find the email address might have changed somehow from what you ordinarily receive.
2. Or possibly somehow your spam-catcher built into your email service has been changed, accidentally blocking this user.
3. there's also a possibility that the email itself that they forwarded to you has a subject that has a spam link, but the actual message is not spam.
What happens if you attempt to open the email?