I know that you have changed the beaviour, but… have you read my previous answer?
Anyway, I think that a more accurate translation for “impossible d’écrire dans le flux” is “Unable to write to stream”.
Is Outlook installed on customer computer? The mail codeunit (id 397) use Outlook to send e-mail. You can use SMTP Mail (codeunit 400) to use a SMTP server to send email and doesn’t need Outlook. To use this codeunit you need to configure “SMTP Mail Setup”.