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What is port 25 and how can it affect email delivery?

Port 25 (SMTP): what it is, what it's for, and how it relates to email within a hosting service.

Glossary

Quick summary

  • What it is: Port 25 is the standard TCP port used by the SMTP (Simple Mail Transfer Protocol) protocol for email transmission between servers.
  • What it's for: it helps configure, send, receive and protect email for a domain.
  • When to check it: when creating email accounts, configuring an email client or investigating sending and receiving issues.

Port 25 is the standard TCP port used by the SMTP (Simple Mail Transfer Protocol) protocol for email transmission between servers. It is the historical channel through which mail servers communicate with each other to deliver messages.

Why is port 25 blocked in shared hosting?

Most shared hosting providers block outbound port 25 for their customers. The main reason is to prevent mass spam sending: if a compromised or misconfigured script could send emails directly through port 25, it could damage the reputation of the entire shared IP and put all server users on blacklists.

Alternatives to port 25

  • Port 587 (Submission): This is the recommended port for sending email from email clients (Outlook, Thunderbird, etc.) through an outgoing server. Requires authentication.
  • Port 465 (SMTPS): Alternative port for SMTP with SSL encryption, although its official use was revoked and port 587 is the currently recommended standard.
  • Port 25 between servers: Continues to be the standard port for server-to-server communication, but end clients do not use it to send email.

When will you encounter it?

Port 25 blocking appears when trying to configure your own mail server from shared hosting, when using PHP scripts to send mass emails, or when configuring applications that try to send email directly without going through the provider's SMTP server. On a VPS or dedicated server you usually have full control over which ports are open.

Why it matters in hosting

Understanding this concept will help you make better decisions when managing your service. In practice, it relates to configuring, delivering, receiving and protecting email for a domain. If it appears in a guide, the control panel or a support response, review the context before making changes.

Related articles

  • Webmail
  • IMAP
  • POP3
  • SMTP
  • SPF