In the meantime we have found out with wireshark that the address is sent to SMTP correctly so with the quotation mark after Nomen: "Nescio, Nomen" nomen.nescio@noreply.com. Apparently the SMTP mail address validation rules decide it's better to put everything before the @ in quotation marks?

All google searches turn up with nothing. We are using this same software in various other situations without any such problem. We have no access to the SMTP server but have raised the question at the representatives.