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: email@example.com 250 2.1.0 Ok rcpt to: firstname.lastname@example.org 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.