Thank you so much for reaching out. We may need to see the error to be able to give you some more guidance around this.
Are the emails being sent to a business domain that may be blocking them?
As you won't be able to post your email address or company information on the Community page can you please contact the Support Team to investigate further.
We would like to capture the message your client is seeing that will help us determine if it is file specific. It would be best please to reach out to our friendly support team so we open a case and investigate further. I'd encourage you to click here so we can get the process started.