This has been happening for about a month. It has been condensing some posts down in to a small attachment labeled 'no name'. If you check some of those posts that appear blank you should see the attachment, you can download 'no name' and open it with notepad (or any text editor) and you will see the message people were trying to send.
My theory is that Topica has implemented some new filtering technology to stop abuse of their lists by spammers and it has a problem with emails originating from the large web-based email providers such as gmail, yahoo, etc. I will do a little research and see if I can dig any thing up.