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)