Several times my emails (invoices) sent via the email server are being classified as Spam on the client's email service due to the email@example.com sub address.
Our email service follows the DMARC email authentication policy. See https://dmarc.org/
Unfortunately bulk mail services such as those in online accounting systems can sometimes become blacklisted by an ISP. If you could contact our support and advise what ISP your clients are with, we can investigate.