SMTP

Detailed explanation

SMTP is the protocol that hands an email off from the sender's mail server to the recipient's mail server, or to intermediate relays along the way. In email marketing, every campaign — whether sent through a dedicated ESP like Mailchimp or a transactional service like SendGrid — ultimately goes out over an SMTP connection, even though the sender rarely interacts with the protocol directly.

SMTP itself does not guarantee inbox placement. Deliverability depends on how the sending domain is authenticated on top of SMTP: SPF and DKIM records that prove the message was not forged, a DMARC policy that tells receiving servers what to do with unauthenticated mail, and a sending IP/domain reputation built up through consistent volume and low complaint rates. Poor authentication or a spammy sending history routes technically valid SMTP mail straight to spam.

For “what is SMTP” or “why are my emails going to spam” searches, this entry is a starting point. See Email Marketing for the broader deliverability and campaign context this protocol sits inside.