The message formatting issue is interesting. I commented about the Yahoo sending limits because it's conceivable that Yahoo is truncating (reducing the length) messages when there are many recipients.
I wonder if you are sending using Yahoo groups using one of the techniques described in this Yahoo help post?
https://help.yahoo.com/kb/groups/pos...p-sln2480.html
If you are sending anything other than a plain text message, there are always things which can go wrong. Without seeing an example of the same message as received properly (at a Yahoo address) and improperly (at another address), it's hard to say more.
Bill