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SMTP (Simple Mail Transfer Protocol) is the internet protocol used to send email and pass it between servers. When you send an email from Outlook, or your website's contact form sends you a notification, the message is delivered to the recipient's mail server via SMTP.
In this article we explain how SMTP works, SMTP server and port settings, common sending errors and what to do so your emails don't end up in the spam folder.
How Does SMTP Work?
- Your email client (Outlook, Thunderbird or a web application) hands the message to its own outgoing mail (SMTP) server using a user name and password.
- The sending server looks up the MX record of the recipient's domain in DNS and finds the recipient's server.
- The message is passed to the recipient's server via SMTP. The receiving server checks whether the sender is authorised with SPF, DKIM and DMARC.
- Accepted email is placed in the recipient's mailbox; the recipient reads it with IMAP or POP3.
SMTP vs IMAP vs POP3
| Protocol | Role | Common ports |
|---|---|---|
| SMTP | Sending email and relaying it between servers | 587, 465, 25 |
| IMAP | Reading email from several devices while keeping it on the server | 993 (SSL) |
| POP3 | Reading email by downloading it to a device | 995 (SSL) |
SMTP Ports: 25, 465 and 587
- 587 (STARTTLS): the recommended submission port for email clients and applications. The connection is encrypted with STARTTLS and authenticated.
- 465 (SSL/TLS): a submission port where the connection is encrypted from the start; supported by many providers.
- 25: used for server-to-server relay. To prevent spam, many ISPs and cloud providers block users from sending out on port 25.
How to Configure SMTP
Email clients and applications usually ask for the following:
| Field | Example value |
|---|---|
| SMTP server (host) | mail.yourdomain.com |
| Port | 587 (STARTTLS) or 465 (SSL/TLS) |
| Authentication | On |
| User name | Full email address (info@yourdomain.com) |
| Password | Your email account's password |
Get the correct server name and port from your email provider's panel or support documentation.
SMTP for WordPress and Websites
By default, websites send email with the server's PHP mail function. Form notifications sent this way have weak authentication, so they often land in spam or never arrive. The fix is to configure the site with an SMTP plugin so it sends through a real email account's SMTP details.
Common SMTP Errors
- 535 Authentication failed: the user name or password is wrong; make sure the user name is the full email address.
- Connection timed out: the port may be blocked; try 587 or 465.
- 550 Relaying denied / Sender not allowed: the server doesn't allow sending without authentication or on behalf of an unauthorised address.
- 554 / rejected as spam: the sending IP address may be blacklisted or SPF/DKIM records may be missing.
Keeping Your Email Out of Spam: SPF, DKIM and DMARC
- SPF: lists in DNS which servers are allowed to send email on behalf of your domain.
- DKIM: adds a digital signature to outgoing email so the recipient can verify the message wasn't altered on the way.
- DMARC: defines what to do with email that fails the SPF and DKIM checks (monitor, quarantine, reject) and lets you receive reports.
Large mailbox providers expect these records to be configured correctly, especially for domains that send in bulk.
What Is a Mail Relay (SMTP Relay)?
A mail relay lets you send your email through an SMTP gateway dedicated to sending instead of through your own server. Your web server's IP address isn't affected by bulk sending and deliverability is easier to manage. It suits sites that send invoices, orders and notifications, and companies that run their own mail server.
ViHyper Business Email and Mail Relay
- Business email plans offer addresses on your own domain, unlimited accounts, antivirus and anti-spam filters, and IMAP, POP3, SMTP and webmail access.
- Mail relay plans let you send over SMTP (TLS) with your own server and software. Your SPF, DKIM and DMARC records are configured during setup. Plans are based on monthly sending volume and may only be used to send to permission-based (opt-in) lists.
For the general benefits of business email, see our article What is business email.
Frequently Asked Questions
Can I use an SMTP server for free?
The SMTP servers of free accounts such as Gmail and Outlook can be used for personal purposes, but they have daily sending limits and aren't suitable for sending from your business domain.
Can I send bulk email with SMTP?
Technically yes, but only to recipients who have given permission (opt-in). Unsolicited bulk email leads to legal problems and to your IP address being blacklisted.
Should I use 587 or 465?
Both are secure if your provider supports them. 587 (STARTTLS) is usually recommended; use 465 if your application only supports SSL.
I changed my SMTP password and now I can't send email. Why?
You also need to update the old password saved in your email clients and in your website's SMTP plugin.
