Group replies are being blocked -- seems to be a problem with group footer triggering something
Groups members are having replies rejected with this error: 554 Message not allowed - [PH01] Email not accepted for policy reasons. Please visit https://help.yahoo.com/kb/postmaster/SLN5067.html 
After trial and error, it seems that the group footer in the message being replied to/quoted is triggering this. Doing a reply and deleting the quoted footer works.
This is a YAHOO problem!!!!
My message before were not blocked and they had same footers so why is YAHOO blocking them now???
YAHOO fix YOUR Problem!!!!
Rebel Alliance commented
Heather -- that seemed to be it for us. I thought at first it was graphics or url's in a member's post, but playing around with a subsequent message I only deleted the yahoo group footer in the quoted reply and the same reply went through whereas with the footer it bounced. Our group is higher ed erp software, so again nothing too out there in terms of content.
Heather Lorimer commented
I am having the exact same problem in the small closed group I head (it is cat genetics, so nothing at all controversial). Do you mean that it is the Yahoo group footer itself that is causing the problem?