Fax to Email Problem, or, Just Fax detecion problem, not sure

Hello All,

Problem : the user on the other side uses an standard fax machine to send a fax to a DID, the DID has fax detection set to SIP with an Email address as the destination. The sender receives an “ error sending” , and the fax call itself is not as you would expect from a successful fax call - no analog noise when the fax detection kicks and so on , but the email does end up in the Email box eventually, the sender of course – does not know that.
Where should I snoop first ?
I’m not using any email relay service, just webmin .

thank you