I received the TBB Newsletter into my Gmail account, which I open in Thunderbird, but to my sruprise and perplexity my wife, who does not have access to that account found it in her IPad Inbox under her user name @nnnnn.plus.com.
I have checked the settings on my Thunderbird email client and there is no sign of a filter to forward anything from TBB to her.
I also compared the Message Source detail.The one in my Inbox has more lines at the start, and that in hers has discrepancies at the end, including an extra anti-virus check i.d. (see attachment)