SMTP (Simple Mail Transfer Protocol) is the standard protocol that mail servers use to send email across the internet. It defines how a sender hands a message to a server, how servers hand messages to each other, and how delivery success or failure is reported back. Every email — newsletter, transactional alert, password reset — travels over SMTP at some point on its way to the inbox.
How SMTP works in practice. A client opens a connection to a server on port 587 (submission) or 465 (implicit TLS), authenticates, and issues a short sequence of commands: HELO or EHLO to introduce itself, MAIL FROM to declare the envelope sender, RCPT TO for each recipient, and DATA to transmit the message body. The server replies with three-digit status codes — 250 means accepted, 550 means rejected — and either queues the message for delivery or returns it to the sender.
Why SMTP matters for senders. A reliable SMTP relay is the difference between a campaign that lands in the inbox and one that gets stuck in a queue or returned to sender. Modern senders rarely run their own SMTP server; they use a managed relay that handles authentication (SPF, DKIM, DMARC), IP reputation, throttling and bounce processing automatically. SMTP is the plumbing — everything else is what makes that plumbing trustworthy.
See also
Mailpro and SMTP
A managed SMTP relay, ready to send
Mailpro gives you a production-grade SMTP relay with authentication, IP reputation, throttling and reporting built in. Plug it in and start delivering — no server to operate.