All SMTP servers basically work this way. This can be done using the freeware PuTTY.
Telnet to server & port: (bold types are user input)
220 BY2NAM03FT048.mail.protection.outlook.com Microsoft ESMTP MAIL Service ready at Wed, 3 Oct 2018 23:54:21 +0000
ehlo
500 5.3.3 Unrecognized command '????' [BY2NAM03FT048.eop-NAM03.prod.protection.outlook.com]
ehlo
250-BY2NAM03FT048.mail.protection.outlook.com Hello [69.60.23.131]
250-SIZE 157286400
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-STARTTLS
250-8BITMIME
250-BINARYMIME
250-CHUNKING
250 SMTPUTF8
mail from:<test@mafia.com>
250 2.1.0 Sender OK
rcpt to:<timlyg@yahoo.com>
250 2.1.5 Recipient OK
data
354 Please start mail input.
subject: This is the Subject Line.
this is just a test
.
250 Mail queued for delivery.
quit
Advanced:
For authentication, after establishing telnet connection and entered two "ehlo" commands, use the command: auth login
334 VXNlcm5hbWU6; will prompt username input, enter base64 encoded username, which is usually the full email address.
334 UGFzc3dvcmQ6; is prompted then for password, enter base64 encoded password.
The rest is the same as above.