Sending anonymous messages to your mailing list
Anonymous is tough to do on the Internet.
We do have a web page for each list, where people can post a message.
And combined with another option where you can designate email addresses allowed to post, but will never receive emails, that may do the trick.
Sending A Message From Our Web Page
See this web page for detailed instructions.
They can put any email address they want in the Your Email Address line.
However, typically they must be a subscriber to your mailing list to allow the message to go to your list.