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Port 25SMTP

SMTP (Simple Mail Transfer Protocol) is one of the core protocols used for sending emails across the Internet. It facilitates the reliable transfer of outgoing mail between email servers and from email clients to servers, enabling seamless global communication. Operating primarily over TCP port 25, SMTP ensures that messages are relayed efficiently, serving as the foundation of modern Internet email infrastructure..

inEmailSecurityNetwork Services
transport
tcp

single transport

in transit
cleartext

payload readable on path

assignment
official

registered with iana

risk
4/10

caution

lookups
10,534

rank 417 of 993 · top 42%

Technical Details

what runs on :25

Simple Mail Transfer Protocol (SMTP) is an application layer protocol defined initially in RFC 821 and updated by RFC 5321. It is designed to transfer mail reliably and efficiently between mail servers. SMTP uses a clear text, command-based communication method that defines how messages are submitted, relayed, and forwarded, primarily functioning in a push model where the client initiates the connection to the server.

By default, SMTP operates over TCP port 25 for server-to-server communications. When an email client submits outgoing mail, it usually connects using port 587 with submission extensions for authentication or port 465 if employing legacy SSL encryption. SMTP dictates the format and delivery of messages but does not retrieve emails from servers; protocols like POP3 or IMAP fulfill that role.

SMTP sessions initiate with a handshake sequence involving HELO/EHLO commands, followed by sender and recipient declarations, and finally the DATA phase where the message content, including headers and body, is transmitted. The connection typically closes upon completion with the QUIT command. Extensions such as ESMTP enhance SMTP's capabilities, adding support for authentication, binary MIME, and more.

Security Information

exposure of :25

risk score

4/ 10caution

worth attention. how exposed you are depends on configuration — don't leave it reachable from the internet without a reason.

email averages 3.9 across 42 ports — this one sits 0.1 above.

in transit

cleartext

credentials and payloads are readable by anyone on path

assignment

official

registered with iana for this service — scanners fingerprint it by number

reachable over

tcp

every listening transport is another surface to filter at the edge

security overview

Common Vulnerabilities:

  • Open mail relays that allow unauthenticated message forwarding, exploited for spam distribution.
  • Lack of encryption, leading to interception of sensitive content or credentials by man-in-the-middle attacks.
  • Weak or absent authentication mechanisms susceptible to spoofing and phishing attempts.
  • SMTP header manipulation facilitating email spoofing or social engineering.

Common Mitigations:

  • Properly configuring SMTP servers to prevent unauthorized relaying and implementing strict relay controls.
  • Enforcing encryption using STARTTLS or moving to secure submission ports (465/587) to protect data in transit.
  • Implementing robust authentication (SMTP AUTH) to control server access.
  • Employing anti-spam and anti-phishing solutions, including SPF, DKIM, and DMARC, to validate senders and reduce spoofing.
  • Regularly patching SMTP software to fix known vulnerabilities and reviewing mail logs for anomalous activities.

Related Ports

all 42 in email →

the 8 most looked-up other ports in email — 42 ports carry that label.

portservicerisk
:143IMAPcaution
:995POP3Scaution
:109POP2caution
:2096cPanel SSL Webmailsafe
:110POP3caution
:993IMAPScaution
:1352Lotus Notes RPCcaution
:24Private Mailcaution

risk mix of the 8 listed

  • safe13%
  • caution88%

3 of 8 encrypted