Email Deliverability Troubleshooting: A Step-by-Step Checklist
Diagnose business email delivery issues by checking DNS authentication, message headers, SMTP responses, sender reputation and application configuration.
When business email lands in spam or fails to arrive, changing random DNS records rarely provides a reliable fix. Start by identifying whether the message was rejected, delayed, accepted into a recipient's mailbox or filtered into spam. The exact response and message headers often point toward the next useful check.
Identify the Actual Failure
Collect the sender, recipient, approximate time, sending application and complete bounce message. A rejection during SMTP is different from a message accepted by the receiving server but later filtered. Avoid testing only by sending to one mailbox provider.
Verify MX, SPF, DKIM and DMARC
Confirm MX records point to the intended receiving service. Check that SPF authorizes legitimate senders, DKIM signatures validate and DMARC alignment is correct. Remember that multiple SPF TXT records can invalidate the policy, and DNS changes may take time to be seen by all resolvers.
Inspect SMTP Logs and Message Headers
Server logs can show authentication failures, connection timeouts, recipient rejection, rate limits and remote policy responses. Message headers can show the receiving server's SPF, DKIM and DMARC results. Redact private message content and credentials before sharing diagnostic output.
Check Server Identity and Reputation
For self-hosted mail, verify the sending IP's reverse DNS, forward DNS consistency, TLS configuration and whether the IP is listed on relevant reputation services. A technically valid message can still be filtered because of sender reputation or unusual sending patterns.
Review Website and CRM Sending
Website forms and CRM systems should use authenticated SMTP or a suitable transactional provider. The From address should belong to a domain the sender is authorized to use; put the visitor's address in Reply-To instead of impersonating it.
Make Changes One at a Time
Record the original DNS values and change only the setting supported by the evidence. Retest with multiple recipients, inspect authentication results and monitor logs after each change. If the remote server returns a specific policy code, use it to guide the next step.
