DMARC bounces and SendGrid authentication issues are multifaceted and can arise from configuration errors, third-party issues, and malicious activity. Key causes include incorrect DKIM/SPF setup (DNS records, selector mismatches, alignment issues), problems with SendGrid (shared IPs, un-warmed IPs, SendGrid domains in headers), email forwarding, content modifications, rDNS misconfiguration, and DMARC policy application on subdomains. External factors such as list bombing can also trigger failures. Troubleshooting requires checking authentication headers, ensuring proper DNS settings, warming IPs, and maintaining domain alignment.