Does SendGrid indicate why they are rejecting requests? In my experience, this is generally due to invalid creds, or a “From address” with a domain that does not match your SendGrid account.

So if a default template is being used with a no-reply@fusionauth.io address you may see a runtime error such as a rejected SMTP request.

Otherwise I'd make sure the IP address is in any whitelists, or share anything else that sendgrid logs.