Is this happeing with all your mails or just few? If it's happening with few mails, then may be the emails addresses are not valid. Can you please share a screen shot here to provide us more context.
Hello @DCreativdesk, just wanted to confirm if this was the knowledge base here that you checked with more information? Are you able to send the email outside of HubSpot?
The more information, screenshots, and details you can provide, the better I can advise on the next steps.