General email troubleshooting
When mail fails, the quickest route is usually a structured check from DNS outward, rather than chasing provider-specific error messages first.
Start with the message headers
Look for Authentication-Results, the envelope sender, the DKIM signing domain, return-path details and any clues from the receiving provider.
Check live DNS
Verify that SPF, DKIM and DMARC records exist publicly, are published at the right hostname and do not contain formatting mistakes.
Check alignment and trust
Even when authentication passes, DMARC may fail because the domains do not align. Also review PTR, HELO and whether the sending IP looks consistent with the domain.
Review behavioural signals
If the technical setup is sound, review complaints, bounce rates, engagement, list sourcing, sudden volume jumps and whether the campaign content matches audience expectations.