Andrew Hodgson writes:
I recently had an incident of all gmail.com members being bounced off one email list (around 180 subscribers).
This is almost certainly due to a recent policy change at Gmail, where they are requiring "from alignment"[1] but only for Gmail authors. That is, Gmail does not publish a p=reject policy, but the effect on Gmail recipients (only) is the same as if they did -- they all bounce every time a Gmail user posts.
Mark has a patch in process, search recent archives of this list for a pointer to it.
Instead of the patch, you can set the "munge from" option for all users instead of the option for p=reject domains only. Or you could ban Gmail addresses from posting. ;-)
Footnotes: [1] "From alignment" is the requirement in DMARC for p=reject or p=quarantine policies that there be a valid DKIM signature from the author's email domain (ie, the From address).