hello Michael Martin,
Sorry for late reply to this,
I have checked all your Joomla user email address and Easyblog subscriber, it seems like all yahoo account do not have 3 o's anymore,
is it your user still getting this?
Undelivered Mail Returned to Sender
Hmm, what my re-commanded is can you try do a filter from your mail account, so that when your inbox getting this kind of email notification so the mail system will try to move it to your filter list.
Also can you provide us some screenshot how it look like when you receive this "Undelivered Mail Returned to Sender" from your inbox? Please advise.