Port 25: SMTP
Open SMTP port detected on a publicly accessible host.
Severity: Warning | Port: 25
What is SMTP?
SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending email between mail servers. Port 25 is the default port for server-to-server email delivery. When you send an email, your mail server connects to the recipient's mail server on port 25 to deliver the message.
Why this is a problem
An open SMTP port is expected on a mail server, but if the server is not properly configured, it can become an "open relay" — meaning anyone on the internet can use it to send email. Spammers actively scan for open relays to send bulk spam and phishing emails, which can get your server's IP address blacklisted.
What you should do
- If this host is not a mail server, disable the SMTP service and close port 25
- If it is a mail server, ensure it is not configured as an open relay — it should only accept mail for your domains or from authenticated users
- Verify that SPF, DKIM, and DMARC records are properly configured for your domain
- Monitor mail server logs for signs of abuse