SMTP
Also known as: ESMTP, STARTTLS, email protocol, mail server, port 587
SMTP (Simple Mail Transfer Protocol) is the protocol computers use to send email.
When you hit send, your email client hands the message to an SMTP server, which routes it through the internet to the recipient's mail server. SMTP handles outgoing mail; IMAP or POP3 handles retrieving mail to read.
SMTP, explained at three levels
At home
When you send an email, SMTP is the protocol that actually moves it. Your email app hands your message to an SMTP server (like a digital post office), which figures out where the recipient's email server is and delivers the message there.
At work
Applications that send automated emails (order confirmations, password resets, notifications) use SMTP to send them. Developers configure an SMTP server (or use a service like SendGrid, Postmark, or AWS SES) to handle the actual delivery.
At home
SMTP uses port 25 (server-to-server), 587 (submission -- email clients to server, requires auth), or 465 (SMTPS, deprecated but still used). The protocol is text-based: commands like HELO (greet), MAIL FROM, RCPT TO, DATA, and QUIT. Modern SMTP uses STARTTLS to upgrade plain connections to encrypted ones.
At work
Email deliverability requires three DNS-based authentication records: SPF (lists authorized sending servers for your domain), DKIM (adds a cryptographic signature to messages), and DMARC (tells receiving servers what to do when SPF/DKIM fail). Transactional email services (SendGrid, Mailgun, Postmark, AWS SES) handle these for developers, with dashboards for delivery rates and bounces.
At home
SMTP's delivery model involves MTA (Mail Transfer Agent) hops -- each server receives the mail and queues it for the next. NDR (Non-Delivery Reports) bounce back when delivery fails. Greylisting temporarily rejects unknown senders (a spam defense) -- legitimate SMTP servers retry; spam bots often don't.
At work
BIMI (Brand Indicators for Message Identification) is a newer standard that displays brand logos in email clients when DMARC enforcement is in place and a verified mark certificate is registered. For high-volume sending, IP reputation management is critical -- dedicated sending IPs with 'warm-up' periods build trust with ISPs. Feedback loops from ISPs report spam complaints back to senders for list hygiene.
Real-world examples
At home
Thomas set up a contact form on his website. When someone submits it, his server uses SMTP (via AWS SES) to email him the submission. Configuring SPF and DKIM prevented his notifications from landing in spam.
At work
Nadia's team sends 500,000 transactional emails per month using SendGrid's SMTP relay. Their dashboard shows delivery rates (99.2%), bounce rates, and spam complaint rates -- they monitor these to protect their domain reputation.
Common confusions
- SMTP is the same as email -- it handles everything — SMTP only sends email. Receiving and reading email uses IMAP (keep messages on server, sync across devices) or POP3 (download messages to one device). SMTP is outbound-only.
- You can send email from any server without configuration — Without SPF, DKIM, and DMARC records configured on your domain, most modern mail servers will reject or spam-folder your messages. Email deliverability requires deliberate DNS configuration.
For analysts
Email campaign analytics (open rates, click rates, bounces) are downstream of SMTP delivery. BAs analyzing email marketing performance should understand that delivery rate is the prerequisite metric -- emails that bounce or land in spam never register opens.
History & origin
SMTP was defined in RFC 821 by Jon Postel in 1982, building on earlier email systems from the ARPANET era. It remains one of the internet's foundational protocols, largely unchanged in its core concept despite extensions (ESMTP) adding authentication, encryption (STARTTLS), and spam controls. The open relay problem (any server would forward mail for anyone) drove the development of sender authentication standards: SPF (2006), DKIM (2007), and DMARC (2012).