SmartReach.io validates each of your prospect's emails before the first email is sent to them. It is done to keep your bounce rates low, and help you build a high email reputation (so that more of your emails reach the inbox, not spam).
However, email validation can not be fully accurate because email service providers like gmail, outlook and others make it impossible to validate certain sets of email domains like generic domains (@gmail.com, @outlook.com), and some corporate email domains as well.
If you are 100% sure that an email that is marked as invalid is in-fact a valid email, then you can do this:
Select Campaign -> Errors -> Invalid emails -> go to the email -> click 'Mark As Valid And Force Send'.
By doing this you are marking an email as valid and SmartReach.io will send emails to these addresses anyway.
You can force-send emails to upto 5 invalid-emails every 24 hours. This is enforced to prevent abuse.
If an email hard bounced earlier, then you can not force-send emails to them.