What is SMTP and how does outgoing email work?
What exactly is SMTP?
SMTP is the language mail servers speak to each other to pass on messages. Compare it to the postal service: you drop off your letter at the post office (the outgoing server), and that office makes sure it ends up at the right address, via other offices along the way if necessary. SMTP is the agreed "protocol" that makes that handover go neatly and reliably.
Important: SMTP is only there for sending and passing on mail. To read messages in your mailbox you use a different protocol: IMAP or POP. Read more about that distinction in the difference between IMAP and POP. Together, fetching (IMAP/POP) and sending (SMTP) form the complete traffic of your mailbox.
Why SMTP matters
For you as a user it comes down to one thing: that your mail actually reaches the recipient. SMTP is the link that arranges that, and all sorts of things can happen along the way that you normally don't notice, until it goes wrong.
- Your messages have to be able to leave. If your outgoing server or port setting is wrong, your mail stays stuck in the "outbox" folder. That's one of the most common causes when sending doesn't work.
- You have to be able to prove yourself. Modern SMTP requires signing in (authentication), so not just anyone can mail in your name. That protects your reputation as a sender.
- Your mail has to come across as trusted. Receiving servers check whether a message is really allowed to come from your domain. For this SMTP works together with domain settings like SPF, DKIM and DMARC.
In short: SMTP is the invisible engine under every sent mail. If it's set up correctly, you notice nothing of it, and that's exactly the intention.
How does SMTP work step by step?
Say you send a message to customer@example.be. Under the hood this happens:
- You deliver your message. Your mail program or webmail connects via SMTP to your provider's outgoing mail server and signs in with your details.
- The server accepts it. After a successful sign-in, the outgoing server receives your message, just as the post office accepts your letter.
- The server finds the destination. The outgoing server checks which mail server receives mail for
example.beand, in turn, connects to it via SMTP. - The handover happens. The two servers exchange the message. The receiving server places it in the recipient's mailbox.
- The recipient reads it. That no longer happens with SMTP, but with IMAP or POP, when the recipient opens their mailbox.
The whole process usually takes seconds. If a step fails (the recipient doesn't exist, the server is unreachable), you get an error message back in your inbox.
What to watch out for
When setting up a mail program you'll come across SMTP as the "outgoing server". Pay attention especially to these points:
- The right port. SMTP uses fixed ports. For secure sending, port 587 (with STARTTLS) is the common choice today; port 465 is also used for encrypted connections. Port 25 is for traffic between servers and is often blocked for ordinary users.
- Always encrypted. Switch on SSL/TLS, so your sign-in details and your message don't go across the net readable.
- Sign in with your full address. Use your full email address as the username, not just the part before the @.
- Sending limits exist. Providers cap how much you can send in a short time to counter abuse. For normal use you notice nothing of it; anyone wanting to do a large mailing is better off using a separate service for that.
The exact addresses, ports and settings for your account are in the SMTP settings for outgoing email. And if you just want to quickly look up the meaning, it's in the term SMTP.
And with Mailflux
With Mailflux you usually don't have to worry about SMTP. If you use the webmail at webmail.mailflux.eu, sending is arranged right away: you don't have to set anything up and simply click send. If you'd rather use a mail program on your computer or phone, every common program works with our POP, IMAP and SMTP support; you enter the server settings once and you're done.
Your email runs on your own domain, hosted in Europe and GDPR-compliant, with antivirus, antispam and antimalware in every plan. That way your outgoing mail leaves securely and reliably. Read more about emailing with your own name behind the @ in email on your own domain.
FAQ
Frequently asked questions
What's the difference between SMTP and IMAP?
SMTP sends your outgoing email; IMAP fetches your incoming email and syncs it with your devices. You need both: SMTP to send, IMAP (or POP) to read. They work together in every mail program.
Which SMTP port should I use?
For secure sending use port 587 with STARTTLS, or port 465 for an encrypted connection. Port 25 is intended for traffic between servers and is often blocked for ordinary users. Always switch on SSL/TLS.
Why does my mail stay in the outbox?
Usually the outgoing SMTP setting is wrong: a wrong server, port, or missing sign-in. Check the outgoing server, the port and whether encryption is switched on. In webmail this doesn't come up, because there sending is already arranged for you.
Do I need SMTP if I only use webmail?
Not as something you set up yourself. In webmail, sending is already arranged for you; you simply click send. SMTP settings only come into view when you connect a mail program on your computer or phone. Then you enter the outgoing server and the port once.