Comcast SMTP server blocked by a number of German ISPs

Our emails to many German domains have been rejected recently.  A typical bounced message looks like the following.  Basically, the IP of the ComCast SMTP server (69.252.207.121 in this case) is blacklisted by these ISPs (gmx.de, t-online.de, ... these are majors ISPs in Germany).  Please advice.

Reporting-MTA: dns; comomta-ch2-01v.sys.comcast.net [69.252.207.121]
Received-From-MTA: dns; [50.254.100.58] [50.254.100.58]
Arrival-Date: Mon, 06 Mar 2017 16:57:42 +0000

Final-recipient: rfc822; xxxxxxx@gmx.de
Action: failed
Status: 5.1.1
Diagnostic-Code: smtp;  554 For explanation visit http://postmaster.gmx.com/en/error-messages?ip=69.252.207.121&c=bip
Last-attempt-Date: Mon, 06 Mar 2017 16:57:43 +0000