Although modern email systems primarily use authenticated SMTP clients and APIs, Telnet remains a useful tool for testing SMTP connectivity and diagnosing mail server issues. System administrators and DevOps engineers often use Telnet to verify whether an SMTP server accepts connections and processes email commands correctly.<\/p>\n\n\n\n
This guide explains how to send a test email using Telnet from a remote Linux server.<\/p>\n\n\n\n
Before starting, ensure that:<\/p>\n\n\n\n
To install Telnet:<\/p>\n\n\n\n
sudo apt update\nsudo apt install telnet -y<\/pre>\n\n\n\nRHEL\/CentOS<\/h3>\n\n\n\n
sudo yum install telnet -y<\/pre>\n\n\n\nStep 1: Connect to the SMTP Server<\/h2>\n\n\n\n
telnet smtp.example.com 25<\/pre>\n\n\n\nExample output:<\/p>\n\n\n\n
Trying 192.168.1.100...\nConnected to smtp.example.com.\n220 smtp.example.com ESMTP Postfix<\/pre>\n\n\n\nStep 2: Introduce Your Server<\/h2>\n\n\n\n
HELO remote.example.com<\/pre>\n\n\n\nResponse:<\/p>\n\n\n\n
250 smtp.example.com<\/pre>\n\n\n\nStep 3: Specify the Sender<\/h2>\n\n\n\n
MAIL FROM:<sender@example.com><\/pre>\n\n\n\nExpected response:<\/p>\n\n\n\n
250 2.1.0 Ok<\/pre>\n\n\n\nStep 4: Specify the Recipient<\/h2>\n\n\n\n
RCPT TO:<recipient@example.com><\/pre>\n\n\n\nExpected response:<\/p>\n\n\n\n
250 2.1.5 Ok<\/pre>\n\n\n\nStep 5: Start the Email Body<\/h2>\n\n\n\n
DATA<\/pre>\n\n\n\nResponse:<\/p>\n\n\n\n
354 End data with <CR><LF>.<CR><LF><\/pre>\n\n\n\nNow enter the email:<\/p>\n\n\n\n
Subject: SMTP Test Email\n\nHello,\n\nThis is a test email sent using Telnet from a remote Linux server.\n\nRegards,\nSystem Administrator\n.<\/pre>\n\n\n\nThe period (
.<\/code>) on a separate line indicates the end of the message.<\/p>\n\n\n\nExpected response:<\/p>\n\n\n\n
250 2.0.0 Ok: queued as 8A3F112345<\/pre>\n\n\n\nStep 6: Close the Connection<\/h2>\n\n\n\n
QUIT<\/pre>\n\n\n\nResponse:<\/p>\n\n\n\n
221 2.0.0 Bye<\/pre>\n\n\n\nComplete Example<\/h2>\n\n\n\n
telnet smtp.example.com 25\n\nHELO remote.example.com\nMAIL FROM:<sender@example.com>\nRCPT TO:<recipient@example.com>\nDATA\nSubject: SMTP Test\n\nHello,\nThis email was sent using Telnet.\n\nThank you.\n.\nQUIT<\/pre>\n\n\n\nTroubleshooting<\/h2>\n\n\n\n
Connection Refused<\/h3>\n\n\n\n
telnet: Unable to connect to remote host<\/pre>\n\n\n\nCheck:<\/p>\n\n\n\n
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- SMTP service is running.<\/li>\n\n\n\n
- Firewall rules.<\/li>\n\n\n\n
- Security groups.<\/li>\n\n\n\n
- Network routing.<\/li>\n<\/ul>\n\n\n\n
\n\n\n\nRelay Access Denied<\/h3>\n\n\n\n
554 Relay access denied<\/pre>\n\n\n\nThis means your SMTP server does not allow unauthenticated relay. Configure SMTP authentication or whitelist the server.<\/p>\n\n\n\n
\n\n\n\nConnection Timeout<\/h3>\n\n\n\n
Connection timed out<\/pre>\n\n\n\nVerify:<\/p>\n\n\n\n