Why survey emails failed to send?

I sent out my first survey using Zoho Survey and the Email launch solution. Of 800+ recipients, using emails we know are accurate because they're used to log into our platform, 741 emails failed to send. I cannot find any guidance on how to address these emails failing to send.

I know we still need to complete our domain authentication steps per this article but i'm not clear if this will address these failed emails from sending since I'm not clear why these emails are failing:

https://help.zoho.com/portal/en/kb/campaigns/user-guide/settings/domain-authentication/articles/authenticate-my-domain#Domain_authentication_methods

Any clarification on why these emails are failing would be helpful to ensure our next steps are most productive for results.