DNS basics for email
Email relies on DNS more heavily than many people realise. Authentication, routing and trust all depend on records being published correctly and consistently.
MX records
MX records tell senders where inbound mail for a domain should be delivered. They do not control outbound authorisation.
A and PTR records
A records map hostnames to IP addresses. PTR records do the reverse. For outbound mail, PTR and forward DNS should make sense together.
TXT records for authentication
SPF, DKIM and DMARC are commonly published via TXT records. Syntax errors, quoting mistakes and publishing records at the wrong hostname are common causes of failure.
TTL and propagation
Changes are not always seen instantly. DNS propagation and cached resolver data can delay validation, especially after fixing an error.