Gmail says - Be Careful With This Message
When you post a message to the mailing list
We send it from our servers to all of the subscribers of the mailing list, including you.
Your email address is in the From: header
So that everybody on the listserve knows who wrote the email.
Gmail notices that it's from your Gmail address, but that it did not come from their servers
Many times spammers send spam to you, from your own address.
Gmail is trying to be helpful
Unfortunately Artificial Intelligence has not figured out the difference between spam and mailing lists.