I guess that as dilbert pointed out although the .eml extension is not to blame directly, it can be a carrier/container for malicious code to reach your inside systems.

In previous versions of Microsoft Outlook (the classic app), you could view the HTML code of an email by opening the email, ...

The EML format is flexible since it can be opened by several email clients, including Windows Live Mail, Thunderbird, and others. You can now get the Advanced OST to EML Converter from the ...