Why am I getting warnings about spoofing and fraud detection
Traditionally, mailing lists have always had the Sender's email address in the From: header
So people will know who each message is from, without having to open it.
These days, ISP's expect that email for a person will only be sent from their ISP. Mailing lists are an exception to this.
Some ISP's will include a warning in the emails that you send to your mailing list.
This sender failed our fraud detection checks and may not be who they appear to be. Learn about spoofing.
Microsoft Office 365 Email services does this for instance.