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From the Model Mayhem FAQ:

Why am I getting weird emails from people I've never heard of before?
Model Mayhem does not disclose the personal information of our members to any third party unless you have authorized us to do so or if we are required by law.

What you are likely experiencing is someone finding your publicly available Model Mayhem profile and then doing some Googling to see if you have your email address listed somewhere. You could have this email listed directly on your Model Mayhem profile, on your personal website, or, perhaps your email just happens to be somewhere on the internet.

We recommend checking your social media profiles and websites and ensuring that your personal information, such as your email, are not listed, and that you only give your email out to people you are certain you really want to work with. This should eliminate those spammy emails.