mail sent from another private account of mine is rejected by Zoho as Relaying disallowed

mail sent from another private account of mine is rejected by Zoho as Relaying disallowed

Since recently mail sent to my Zoho account (alias) arx@a-rx.net
from my other private account reuleaux@web.de is rejected with
(Relaying disallowed), cf the returned message below.
I am the legal/honest owner of both of these accounts and not trying
to do any harm to anybody (but just testing my mail configuration).
That I cannot send e-mail to myself any more is unfortunate, and makes
my Zoho account only so useful (I have send an e-mail yesterday evening
as well explaining this problem, from the contact form of your website,
with no answer so far).

From: < keineantwortadresse@web.de>
Subject: Mail delivery failed: returning message to sender
To: reuleaux@web.de
Date: Thu, 02 Apr 2015 04:35:02 +0200 (9 hours, 13 minutes, 3 seconds ago)
Message-Id: <E1YdUyG-0004OI-6n@varenne>

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of
its recipients. This is a permanent error. The following address
failed:

" arx@a-rx.net":
SMTP error from remote server after RCPT command:
host: mx.zohomail.com
Relaying disallowed


--- The header of the original message is following. ---

Received: from varenne ([81.151.255.196]) by smtp.web.de (mrweb003) with
 ESMTPSA (Nemesis) id 0MRCoR-1Z1S160wAR-00UbWg for < arx@a-rx.net>; Thu, 02 Apr
 2015 04:34:59 +0200
Received: from rx by varenne with local (Exim 4.84)
    (envelope-from < reuleaux@web.de>)
    id 1YdUy6-0004Na-61
    for arx@a-rx.net; Thu, 02 Apr 2015 03:34:54 +0100
From: Andreas Reuleaux < reuleaux@web.de>
To: Andreas Reuleaux < arx@a-rx.net>
Subject: test from web.de to z
Date: Thu, 02 Apr 2015 03:34:54 +0100
Message-ID: < 87mw2r8dgx.fsf@web.de>
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