Formitize has a spam filter in place, which will stop an email from sending if the filter comes across something in the email message that has a high chance of being caught in receivers' spam folders when being sent to them.
If you would like to bypass the filter, then you may contact Support via the widget found in the help menu in your account. Please be aware that this heightens the chance that your invoices and quotes will be sorted into customer spam folders, and that you may have to follow up with them.