I have the same problem with SMTP. Test works fine, but actual form won't.
When using barber template the form builder addon won't work, but with the default Joomla template form buidler addon works fine.
After pressing the sendbutton the form content show up in the page URL.
?form-builder-item-%5Bfirst-name%5D=Name1&form-builder-item-%5Blast-name%5D=Name2&form-builder-item-%5Bemail%5D=test%40email.com&form-builder-item-%5Bsubject%5D=Test+message&form-builder-item-%5Bmessage%5D=Testing.&form_id=eyJyZWNpcGllbnRfZW1haWwiOiJjR2x5YVhSMFlVQndkWFZ6WlhCaGJteHBhV3RsY21GellXNWxiaTVtYVE9PSIsImFkZGl0aW9uYWxfaGVhZGVyIjoiVW1Wd2JIa3RWRzg2SUh0N1pXMWhhV3g5ZlFwU1pYQnNlUzF1WVcxbE9pQjdlMlpwY25OMExXNWhiV1Y5ZlNCN2UyeGhjM1F0Ym1GdFpYMTlDZz09IiwiZnJvbSI6ImNHbHlhWFIwWVVCd2RYVnpaWEJoYm14cGFXdGxjbUZ6WVc1bGJpNW1hUT09In0%3D%3Aea4c5aae0b04c565b9a5e1d71f512a7a&addon_id=1589708819691&email_subject=e3tzdWJqZWN0fX0gfCBZaHRleWRlbm90dG8gLyBTw6RpbHl0eXMgamEgbGl1a3VvdmV0IHwgUFNMIEouIFLDpHPDpG5lbg%3D%3D&email_template=PHA%2BPHN0cm9uZz5Mw6RoZXR0w6Rqw6Q6IDwvc3Ryb25nPnt7Zmlyc3QtbmFtZX19IHt7bGFzdC1uYW1lfX08L3A%2BCjxwPjxzdHJvbmc%2BU8OkaGvDtnBvc3Rpb3NvaXRlOiA8L3N0cm9uZz57e2VtYWlsfX08L3A%2BCjxwPjxzdHJvbmc%2BQWloZTogPC9zdHJvbmc%2Be3tzdWJqZWN0fX08L3A%2BCjxwPjxzdHJvbmc%2BVmllc3RpOiA8L3N0cm9uZz57e21lc3NhZ2V9fTwvcD4%3D&success_message=T2RvdGEgaGV0a2ku&failed_message=Vmllc3RpbiBsw6RoZXR5cyBlcMOkb25uaXN0dWkuIFlyaXTDpCB1dWRlbGxlZW4u&captcha_type=default