I am also having this issue with one client.  QBO has been saying for months that this client's email is bouncing but the client assures me that they are receiving the invoices.  I can confirm that they are viewing them. Currently, this is happening with only one client who recently updated their email address.  I do not know if this contributed to the issue. I hope this gets resolved soon. Please update the description of this thread as "Under Investigation" not "Solved."