Unable to send email to our club members alias

I recently became unable to send email to club aliases. Has MailChannels.net been blocked? Part of bounce message copied below.

Thanks,
Paul
Club #3491 - eastsidetoastmasters.org

<members@eastsidetoastmasters.org>: host
mail.eastsidetoastmasters.org[50.19.253.65] refused to talk to me: 421
Downstream server error
Reporting-MTA: dns; relay.mailchannels.net
X-Postfix-Queue-ID: 07AC6641D4C
Arrival-Date: Thu, 17 Dec 2020 15:51:22 +0000 (UTC)

we are not blocking any of the 4 reporting IP addresses

A-->mx1.mailchannels.net 44.241.163.138
A-->mx1.mailchannels.net 44.230.102.231
A-->mx2.mailchannels.net 34.218.143.220
A-->mx2.mailchannels.net 34.211.70.105

Thanks for your quick response. What about relay.mailchannels.net? Any other ideas why I am unable to send email to the members alias? (I checked with another member, and he was able to send to the alias successfully.)

That does not post as a valid email domain for your email address.
It is not listed as one of your valid MX records.

we are not blocking
relay.mailchannels.net. 300 IN A 216.19.178.99

You need to post the entire email header if you want me to look further.

Here are the full headers:

<members@eastsidetoastmasters.org>: host
mail.eastsidetoastmasters.org[50.19.253.65] refused to talk to me: 421
Downstream server error
Reporting-MTA: dns; relay.mailchannels.net
X-Postfix-Queue-ID: 07AC6641D4C
X-Postfix-Sender: rfc822; paul@kromann.net
Arrival-Date: Thu, 17 Dec 2020 15:51:22 +0000 (UTC)

Final-Recipient: rfc822; members@eastsidetoastmasters.org
Original-Recipient: rfc822;members@eastsidetoastmasters.org
Action: failed
Status: 4.0.0
Remote-MTA: dns; mail.eastsidetoastmasters.org
Diagnostic-Code: smtp; 421 Downstream server error
Return-Path: <paul@kromann.net>
Received: from relay.mailchannels.net (localhost [127.0.0.1])
by relay.mailchannels.net (Postfix) with ESMTP id 07AC6641D4C
for <members@eastsidetoastmasters.org>; Thu, 17 Dec 2020 15:51:22 +0000 (UTC)
Received: from pdx1-sub0-mail-a27.g.dreamhost.com (100-105-161-17.trex.outbound.svc.cluster.local [100.105.161.17])
(Authenticated sender: dreamhost)
by relay.mailchannels.net (Postfix) with ESMTPA id 5233F641D4A
for <members@eastsidetoastmasters.org>; Thu, 17 Dec 2020 15:51:21 +0000 (UTC)
X-Sender-Id: dreamhost|x-authsender|paul@kromann.net
Received: from pdx1-sub0-mail-a27.g.dreamhost.com (pop.dreamhost.com
[64.90.62.162])
(using TLSv1.2 with cipher DHE-RSA-AES256-GCM-SHA384)
by 0.0.0.0:2500 (trex/5.18.11);
Thu, 17 Dec 2020 15:51:21 +0000
X-MC-Relay: Neutral
X-MailChannels-SenderId: dreamhost|x-authsender|paul@kromann.net
X-MailChannels-Auth-Id: dreamhost
X-Industry-Tank: 26761cec3e4b6d04_1608220281592_3543373839
X-MC-Loop-Signature: 1608220281592:2558179006
X-MC-Ingress-Time: 1608220281592
Received: from pdx1-sub0-mail-a27.g.dreamhost.com (localhost [127.0.0.1])
by pdx1-sub0-mail-a27.g.dreamhost.com (Postfix) with ESMTP id 9DEF87F6A3
for <members@eastsidetoastmasters.org>; Thu, 17 Dec 2020 07:51:20 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=kromann.net; h=from
:content-type:mime-version:subject:message-id:date:to; s=
kromann.net; bh=J++MlwO5GyZ/705f8W2y1dZP2Og=; b=G33FNfchUHhEb49m
DQfJafmGMpQo6butFsOR6JwhcUvHmSqVNEs6U7H47BXXWnnwSznqNd+ANi6ixDrR
ULzTL9LnaQvlsnHE+sn3+eZq5+/vhXggcVl7cHE4GElwvEGyJAWIHqtoNoURDJ/U
zZtus2hnVmakc4XUJeJBHFh8Gxg=
Received: from [192.168.1.13] (unknown [50.35.71.179])
(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
(No client certificate requested)
(Authenticated sender: paul@kromann.net)
by pdx1-sub0-mail-a27.g.dreamhost.com (Postfix) with ESMTPSA id 877CC7F113
for <members@eastsidetoastmasters.org>; Thu, 17 Dec 2020 07:51:20 -0800 (PST)
X-DH-BACKEND: pdx1-sub0-mail-a27
From: Paul Kromann <paul@kromann.net>
Content-Type: multipart/alternative;
boundary="Apple-Mail=_A718419B-509E-4FC7-87E1-5CCA59B939E4"
Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.40.0.2.32\))
Subject: Reminder: Prepare for tonight's Winter Holiday Party!
Message-Id: <9502F890-3054-4684-A345-908705A6FA38@kromann.net>
Date: Thu, 17 Dec 2020 07:51:18 -0800
To: members@eastsidetoastmasters.org
X-Mailer: Apple Mail (2.3654.40.0.2.32)

Also, my understanding is that the MX records for kromann.net are / should be:

mx1.mailchannels.net
mx2.mailchannels.net

And an SPF TXT record:

v=spf1 mx include:netblocks.dreamhost.com include:relay.mailchannels.net -all

I'm not an expert in mail routing, but my understanding is that this should indicate relay.mailchannels.net as a valid server for kromann.net email.

I have successfully sent email to FTH aliases over the past 6 months. Now something suddenly changed within the past week so that I can no longer send mail using this address. Any idea why my email is being rejected?