SMTP relay
Sender MTA (Sending Mail Transfer Agent) opens TCP to recipient MTA. HELO, MAIL FROM, RCPT TO, DATA. Text protocol from the 80s.
SMTP relay
Sender MTA (Sending Mail Transfer Agent) opens TCP to recipient MTA. HELO, MAIL FROM, RCPT TO, DATA. Text protocol from the 80s.
SPF
DNS TXT record listing IPs allowed to send from your domain. Recipient checks: is the sending IP in the SPF list?
DKIM
Sender signs message headers + body with private key. DNS TXT publishes public key. Recipient verifies signature — proves message wasn't tampered.
DMARC
Ties SPF + DKIM to your domain. Policy: what to do on failure (none / quarantine / reject). Publishes reports to sender for monitoring.
Reputation trumps rules
Even with perfect SPF/DKIM/DMARC, new sender IPs get filtered. Warm up gradually. Volume + engagement + spam complaints build reputation.