Connect to the mail.foo.bar on port 25
$ nc mail.foo.bar 25
220 mail.foo.bar ESMTP Postfix
Once connected, say hello to the email server
$ nc my.email.server 25 220 mail.foo.bar ESMTP Postfix
HELO foo.bar250 mail.foo.bar
Specify a return address, just in case the email bounced. This is an SMTP protocol requirement.
$ nc my.email.server 25 220 mail.foo.bar ESMTP Postfix
HELO foo.bar
250 mail.foo.bar
MAIL FROM: mrbean@foo.bar
250 2.1.0 Ok
Specify recipient of the email.
$ nc my.email.server 25 220 mail.foo.bar ESMTP Postfix
HELO foo.bar
250 mail.foo.bar
MAIL FROM: mrbean@foo.bar
250 2.1.0 Ok
RCPT TO: teddy@foo.bar
250 2.1.5 Ok
Put in you message (any message will do, but below is just how normal email usually formatted). Once you are done typing in your message, put dot (.) as a signal you are done with the DATA, and server will reply with your queue number.
$ nc my.email.server 25 220 mail.foo.bar ESMTP Postfix
HELO foo.bar
250 mail.foo.bar
MAIL FROM: mrbean@foo.bar
250 2.1.0 Ok
RCPT TO: teddy@foo.bar
250 2.1.5 Ok
DATA
354 End data with. From: mrbean@foo.bar
To: teddy@foo.bar
Subject: A gift
Date: Thu, 22 Mar 2018 12:00:00 +0000
This is merely a gift for you. Enjoy.
.
250 2.0.0 Ok: queued as 7F571241267C
To exit, just type quit, and you are done.
$ nc my.email.server 25 220 mail.foo.bar ESMTP Postfix
HELO foo.bar
250 mail.foo.bar
MAIL FROM: mrbean@foo.bar
250 2.1.0 Ok
RCPT TO: teddy@foo.bar
250 2.1.5 Ok
DATA
354 End data withFrom: mrbean@foo.bar .
To: teddy@foo.bar
Subject: A gift
Date: Thu, 22 Mar 2018 12:00:00 +0000
This is merely a gift for you. Enjoy.
.
250 2.0.0 Ok: queued as 7F571241267C
QUIT
221 2.0.0 Bye
That's all, the recipient should already received the email, or you should get a return email if it is bounced.