What is SMTP? A Clear Guide

SMTP (Simple Mail Transfer Protocol) is the standard used to send email between servers. Understanding it helps you diagnose delivery, authentication and relay problems.

Why it matters

Knowing SMTP helps you send mail that lands in the inbox. See the deliverability checklist.

How Rainmail helps

Rainmail sends from your own domain on a dedicated, isolated IP with SPF, DKIM, DMARC, reverse-DNS and managed warm-up handled for you, plus an AI watching list health and deliverability — and it welcomes senders SES, SendGrid and Mailchimp reject, from $8. Run the free checker or see plans.