When sending a digitally signed email with a certificate, the fax is rejected due to the smime.p7s attachment. This article provides details on how to overcome this issue and complete sending the fax.
This is an example of such a message:
P7s files are processed like any other attachment received by our system.
The system is not set up to parse which attachments are processed and which ones are not, so when an unaccepted file type is received, such as a p7s file, the system is unable to process and fails the request.
The outbound faxes need to be sent unsigned to be processed.
There are no file exceptions that can be processed at this time.