Just my 2c.

On Mon, 16 Dec 2019 07:51:40 -0500
Haines Brown <haines@???> wrote:

> 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.

That corresponds with the frequency of messages to the list.

> 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@???: "Dng"
> > <dng-bounces@???> 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?

Correct. what mailer are you running and what setting are you using
to cull empty sender messages.
>Why is dng-bounces sending me messages?
Probably because all messages I receive from the lists have
Sender: "Dng" <dng-bounces@???>

as the sender. This is commonly set on lists so that bounce messages,
and other automated replies(out of office anyone?) do not pollute the
list. Admins can then go through the bounces and take appropriate
action.
So, in summary, it is a setting in your mailer that creates the error
messages. As a guess you need to relax a time parameter in that your
mailer is closing receipt of header too quickly or the link is dropping
out.