If you have the ‘From’ address as the clients email address ensure you have added the appropriate values to their SPF record for that email domain.
This can be done irrespective of the site being live or in trial.
Documentation for SPF record settings is here:
https://docs.treepl.co/documentation_group/treepl-portal/treepl-cms-hosted-email#secSPFRecords
And a similar conversation which may help:
Form autoresponders going to spam?