I spent a few frustrating evenings recently trying to figure out why sending email from a WordPress site was failing when sending to a Google Group. It worked fine for a very long time, and then suddenly started failing, rejected with the vague message “nested groups detected”. There’s no information from Google about what it considers nested groups and what to do about it.
Eventually I narrowed it down to the “Precedence” header. Apparently, if Google Groups sees this header, it determines this is already being sent through a list and refuses to send it.
The solution was to remove the header and in my case, commenting out a line in the otherwise excellent Subscribe2 WordPress plugin. I commented out this line in class-s2-core.php:
$header['Precedence'] = "list\nList-Id: " . html_entity_decode(get_option('blogname'), ENT_QUOTES) . "";
I posted the solution in the WordPress forum and hopefully the author will add an option for it.