The Simple Mail Transfer Protocol (SMTP) is the standard set of rules that email servers use to send and relay messages across the internet. Every time an email leaves a mailbox, SMTP is what carries it from one server to the next until it reaches its destination. It handles the sending side of email specifically; other protocols deal with reading what arrives.
Think of SMTP as the postal sorting and delivery network, not the letterbox you read mail from. You drop a letter in the box (you hit send), and a chain of sorting offices passes it along until it reaches the right address. This is why it matters for websites: when a smart form or contact form is submitted, the site has to send that data as an email, and it does so by handing the message to an SMTP server. Like HTTP for web pages, SMTP is one of the quiet protocols the internet runs on.
If SMTP is misconfigured, those notification emails simply never arrive, which is a surprisingly common cause of missed leads. The trickier failure is silent: the mail technically sends, but lands in spam because the sending domain lacks the right records (SPF, DKIM, DMARC) that prove it is allowed to send on your behalf. That is why most teams now route form mail through a dedicated provider like Postmark or SendGrid rather than a raw server. A provider like that also gives you delivery logs, so when someone insists they never got the email, you can actually see whether it left and where it went rather than guessing.
At TopDevs we set up reliable SMTP delivery on every site with forms, so the messages your visitors send actually reach your inbox.
