Hi Jason,
After checking the email address you send your customer emails from, on your account, the domain information comes back stating that Clear Books is not authorised to send emails on your behalf. The guide here https://www.clearbooks.co.uk/support/guides/emails/ensuring-your-clear-books-emails-are-not-rejected-by-recipients/ will show you what you need to do to correct this. Once the DNS record has been updated, the emails should stop being marked as spam. It may also be worth checking that customers have you added as a contact as well, as this may also help.
If you need any further assistance, please do let us know.
Kind Regards,
Natasha