I periodically get frozen messages alerts from dng-bounces. Sometimes
several in a day; sometimes every few days. The time of the message is
random. I'm runing beowulf. Here is an example

> Date: Sun, 15 Dec 2019 13:48:21 -0500
> From: Mail Delivery System <Mailer-Daemon@???>
> To: postmaster@???
> Subject: Message frozen
>
> Message 1igYwF-0002qx-Da has been frozen (delivery error message).
> The sender is <>.
>
> The following address(es) have yet to be delivered:
> dng-bounces@???: SMTP error from remote mail server
> after
> pipelined
> +MAIL FROM:<> SIZE=5953: 554 5.7.1 Empty Sender Address is
> prohibited
> through this server

Is my mailer telling me it has frozen a message from dng-bounces
because its FROM: field is empty? Why is dng-bounces sending me
messages?

Haines Brown