Problem on sending email to some domain - Error: 451 Temporary local problem, please try again

Hello,

we are sending out our emails from our internal mailserver.
For some email domains we can everytime a local mail error:

------------
Sophos XG Firewall was unable to send the following mail:

From: d.xxxx@bixxxxxx.de
MessageID: <f04d4d62-48ca-6740-609f-a011906ba128@bixxxxxx.de>
Sent on: 2021-01-07 16:54:24

Mail delivery to following recipients failed:

yyy@mohn-gmbh.com - 451 Temporary local problem, please try again!


DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
	d=binsack-reedtechnik.de; s=mail;
	h=from:subject:date:message-id:to:mime-version:content-type:
	 content-transfer-encoding:in-reply-to:references;
	bh=ram0cpB6DP6WKSqulhSz2oiHpsPc0XDwtm4LPikecdg=;

X-Footer: Ymluc2Fjay1yZWVkdGVjaG5pay5kZQ==
Received: from ....
	(authenticated user d.xxxx@bixxxxxx.de ) 

by mail.bixxxxxx.de with ESMTPSA (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256 bits))

------------

This error occur also by more special domains. But we can receive mails from this domains.

Other domains function well.

When I look in protocol view, everything seems fine.

We use actually no  MTA- modus.

So there will be no greylisting active .  or ?

Our system: 

Sopos XG 115
FW: SF 17.5.14 MR-14-1

What should I do ?

Thanks

best regards

Dirk Mühlhauser