You need some way to report suspisous emails outside of premium service. Preventing this is just as harmful to your premium clients.
I received a suspicious looking email from yahoo, and have no way to report it. This feedback feature is the only way I have found to even mention it outside of premium service. I am not paying for a premium service to warn yahoo that customer base might be under attack.
Preventing us from reporting suspicious emails hurts your premium members as much as it hurts the rest of us.

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HI Michael, you do not need to send us a malware or suspicious looking email. Unfortunately those emails are out there, going to all mail clients including Gmail, Outlook, Hotmail, etc. We recommend not providing those phishing emails with any personal information, don't click on any links and mark it as spam. It will prevent that email from going into your Inbox. Thanks!