Actually, one of our customers found out the hard way that there is harm in sending emails to invalid addresses. Too many kickbacks and cloud services think you’re a bot. Prevented the customer from being able to send emails for 24 hours.
This is the result of them “requiring” an email for customers but entering a fake one if they didn’t want to provide their email, and then trying to send out an email to everyone.
Our software has an option to disable that requirement but they didn’t want to use it because they wanted their staff to remember to ask for an email address. It was not a great setup but they only had themselves to blame.