Telnet can be used to send an email. Using telnet is generally frowned upon, as there are much easier and much more secure ways to send email. However, if an email must be sent using telnet, the telnet hostname port command can be used to send an email.

In this example, an email is sent from a Windows computer using the Command Line Prompt or PowerShell.

C:\Users\desktop> telnet mail.example.com 25
220 mail.example.com 
ESMTP Postfix
EHLO mail.example.com
250-mail.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
mail from: john.doe@example.com
250 2.1.0 Ok
rcpt to: jane.doe@example.com
250 2.1.5 Ok
data
354 End data with <CR><LF>.<CR><LF>
Hello World
.
250 2.0.0 Ok: queued as 188B012006D
quit
221 2.0.0 Bye
Connection to host lost.