1) "no bounces/error messages", and "If I change the return email address to any other they are received." - I reckon that means the system is working?
2) Return addresses are correct and WERE working normally 3 days ago
3) I've tried more than one smtp server. The domain host does not offer a (free) smtp server but the ones I have been using have been working fine - up to 3 days ago.
I understand the system looks up the return address to validate it? Could it be a DNS disfunction at her domain host or would I expect a rejection message from the server?