Confirm to bug subscription doesn't work

[Felix Natter <fnatter@gmx.net>]

hello mentors,

I tried to confirm subscription to #843702, but my mailer (Emacs/Gnus)
fails with:
Sending via mail...
235 Authentication succeeded
250 Requested mail action okay, completed
501 Syntax error in parameters or arguments
221 gmx.com Service closing transmission channel

Here is the reply that my mailer generated:

From: Felix Natter <fnatter@gmx.net>
To: 843702-subyes-fc34d13d0780cb12c6f8ff28d9b6f8df-3bce6c49747938afe157a393c43e5732@bugs.debian.org
Subject: Re: Please confirm subscription to 843702@bugs.debian.org (freeplane should add standard MIME open for .mm files)
References: <20161115155713.4ABA41D8@bendel.debian.org>
X-Draft-From: ("nnml:mail.misc" 69494)
Gcc: nnfolder+sent:sent-mail.2016-11
Date: Tue, 15 Nov 2016 17:02:27 +0100
In-Reply-To: <20161115155713.4ABA41D8@bendel.debian.org>
	(843702-subhelp@bugs.debian.org's message of "Tue, 15 Nov 2016 15:57:13
	+0000 (UTC)")
Message-ID: <8760no91f0.fsf@bitburger.home.felix>
User-Agent: Gnus/5.130006 (Ma Gnus v0.6) Emacs/24.4 (gnu/linux)
--text follows this line--
[...]

Does anyone know this bug or is it a Gnus issue?

Many Thanks and Best Regards,
-- 
Felix Natter