SMTP relay
Sender MTA (Sending Mail Transfer Agent) opens TCP to recipient MTA. HELO, MAIL FROM, RCPT TO, DATA. Text protocol from the 80s.
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SMTP relay
Sender MTA (Sending Mail Transfer Agent) opens TCP to recipient MTA. HELO, MAIL FROM, RCPT TO, DATA. Text protocol from the 80s.
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SPF
DNS TXT record listing IPs allowed to send from your domain. Recipient checks: is the sending IP in the SPF list?