Internet email support - cannot send

Comcast.net is blocking our domain (lechler.de) from sending emails.  After 4 hours there is a delay message. Then at 3 days there is a rejection message.  See below:

(Delay Message)

    ----- Transcript of session follows -----
451 4.4.1 reply: read error from mx2.mxge.comcast.net.
451 4.4.1 reply: read error from mx1.mxge.comcast.net.
451 4.4.1 reply: read error from mx1c1.comcast.net.
451 4.4.1 reply: read error from mx2c1.comcast.net.
451 4.4.1 reply: read error from mx1a1.comcast.net.
451 4.4.1 reply: read error from mx2h1.comcast.net.
451 4.4.1 reply: read error from mx1h1.comcast.net.
451 4.4.1 reply: read error from mx2a1.comcast.net.
<****@comcast.net>,<****@comcast.net>,<****@comcast.net>,<****@comcast.net>... Deferred: Connection reset by mx2a1.comcast.net.

Warning: message still undelivered after 4 hours
Will keep trying until message is 3 days old

(Failure Message)

    ----- The following addresses had permanent fatal errors ----- <****@comcast.net>

    ----- Transcript of session follows -----

451 4.4.1 reply: read error from mx2.mxge.comcast.net.

451 4.4.1 reply: read error from mx1.mxge.comcast.net.

451 4.4.1 reply: read error from mx2a1.comcast.net.

451 4.4.1 reply: read error from mx2c1.comcast.net.

451 4.4.1 reply: read error from mx1a1.comcast.net.

451 4.4.1 reply: read error from mx1h1.comcast.net.

451 4.4.1 reply: read error from mx1c1.comcast.net.

451 4.4.1 reply: read error from mx2h1.comcast.net.

<****@comcast.net>,<****@comcast.net>,<****@comcast.net>,<****@comcast.net>... Deferred: Connection reset by mx2h1.comcast.net.

Message could not be delivered for 3 days Message will be deleted from queue

For 20 years we never had an issue sending to comcast.net users and suddenly (about 6 months ago) all of our emails are getting rejected by comcast.net.  Nothing our end has changed.  Comcast says they are not blocking anything and I have confirmed that we are not on any block lists. (No other issues, ever, sending to any other domain.)

Comcast.com (business) is our ISP and we have no issues sending to comcast.com emails.  They say they can't help because Comcast.net a different department.

If we test the connection from behind our firewall to the Comcast mailserver on port 25, the connection is immediately closed the foreign (Comcast) host.

If we ping from outside the firewall with a public IP, the connection is accepted.

I saw a similar post about this but the thread is now closed and there was no solution.

https://forums.xfinity.com/conversations/email/not-receiving-emails-from-a-uk-web-site/640f97c041879c3c4d1c38f2?commentId=64628d1121d18806b4f9cf7a

I have been rudely refused anymore support through their technical support email address and call back.  Can anyone help?