Try this: $ telnet mail.example.org 587 Trying 192.168.1.52... Connected to mail.example.org. Escape character is '^]'. 220 'mail.example.org' E

Try this: $ telnet mail.example.org 587 Trying 192.168.1.52... Connected to mail.example.org. Escape character is '^]'. 220 'mail.example.org' ESMTP Exim 4.92 Mon, 31 May 2021 17:30:49 +0200 mail from: father_christmas@northpole.no 250 OK rcpt to: your_mail_address_here@your_mail_host_here 250 Accepted data 354 Enter message, ending with "." on a line by itself This message will not show a TO address but it will still arrive... . 250 OK id=1lnjuC-0003xa-5H quit 221 'mail.example.org' closing connection Connection closed by foreign host. Now check your mail box: Return-path: <father_christmas@northpole.no> Envelope-to: your_mail_address_here@your_mail_host_here Delivery-date: Mon, 31 May 2021 17:32:59 +0200 Received: from session.example.org ([192.168.1.53]) by 'mail.example.org' with smtp (Exim 4.92) (envelope-from <father_christmas@northpole.no>) id 1lnjuC-0003xa-5H for your_mail_address_here@your_mail_host_here; Mon, 31 May 2021 17:32:59 +0200 Message-Id: <E1lnjuC-0003xa-5H@'mail.example.org'> From: father_christmas@northpole.no Date: Mon, 31 May 2021 17:32:59 +0200 This message will not show a TO address but it will still arrive... That's is, no subject header, no TO header, no nothing but the message still arrives since all that is needed is a sender (mail from:), a recipient (rcpt to:) and some content (data followed by an line containing nothing but a period)