What Is an SMTP Banner?
Understanding SMTP banners and the information they provide
Table of Contents
What Is an SMTP Banner?
An SMTP banner is the initial greeting message sent by an SMTP server when a client connects. It's the first response received after establishing a connection and provides information about the server.
SMTP banners identify the server, provide version information, and may indicate server capabilities. They help verify server identity, troubleshoot connection issues, and understand server configuration.
When you connect to an SMTP server using our SMTP server test tool, the banner is one of the first pieces of information displayed, helping you verify the server and understand its configuration.
Banner Information
Server Identification
SMTP banners typically include the server hostname, which helps identify which server you're connected to. This is useful for verifying you're connecting to the correct server.
Software Information
Banners often include the mail server software name and version (e.g., "Postfix", "Microsoft Exchange", "Sendmail"). This helps identify the server type and version.
Server Capabilities
Some banners include information about server capabilities, such as supported authentication methods, encryption support, or feature flags.
Additional Information
Banners may include additional details like server status, maintenance information, or custom messages configured by administrators.
Banner Format
SMTP banners follow a standard format defined by the SMTP protocol. They typically start with a status code (220) followed by the banner message.
Standard Format
Banners typically follow this format: 220 hostname ESMTP server-software version
Example Banners
220 mail.example.com ESMTP Postfix220 mail.example.com Microsoft ESMTP MAIL Service220 mail.example.com ESMTP Sendmail 8.15.2
Status Codes
Banners use status code 220, which indicates the server is ready to accept commands. Other codes indicate errors or issues.
Using SMTP Banners
Server Verification
Banners help verify you're connected to the correct SMTP server by displaying the server hostname and identification.
Server Type Identification
Banner information helps identify the mail server software, which can be useful for troubleshooting and understanding server behavior.
Configuration Verification
Banners can reveal server configuration details, helping verify settings and identify potential issues.
Connection Testing
Receiving a proper banner indicates the connection was successful and the server is responding. Missing or malformed banners indicate connection problems.
Troubleshooting with Banners
SMTP banners are valuable for troubleshooting connection and configuration issues:
Connection Verification
If you receive a banner, the connection is successful. If no banner is received, there's a connection problem.
Server Identification
Banner hostname helps verify you're connecting to the correct server, not a misconfigured or wrong server.
Error Diagnosis
Banner format and content can reveal server issues, misconfigurations, or problems that affect email delivery.
Testing Tools
Use our SMTP server test tool to view SMTP banners and analyze server responses for troubleshooting.
