550 Access denied - Invalid HELO name (See RFC5321 4.1.1.1)

Hi, I'm not a regular forum user but this time I had no other option. I'm facing this problem since I migrate from shared hosting to a VPS with WHM / Cpanel and sometimes, just in SOME HOSTS, when i send an email, i get this error from from my own VPS, by "Mail delivery System" [PHP] marketing@pinga.pt SMTP error from remote mail server after MAIL FROM: SIZE=14077: host vps46790.ovh.net [37.187.54.234]: 550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1) ------ This is a copy of the message, including all the headers. ------ Return-path: Received: from av3-84-90-82-114.netvisao.pt ([84.90.82.114]:50023 helo=AsusX52J) by vps46048 with esmtpa (Exim 4.82) (envelope-from ) id 1WPu2I-0003Cj-7y for marketing@pinga.pt; Tue, 18 Mar 2014 13:26:30 +0000 From: =?iso-8859-1?Q?Gon=E7alo_Cabral?= To: Subject: teste Date: Tue, 18 Mar 2014 13:27:07 -0000 Message-ID: <045b01cf42ad$c29ae420$47d0ac60$@goncalocabral.com> MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_NextPart_000_045C_01CF42AD.C29BCE80" X-Mailer: Microsoft Outlook 14.0 Content-Language: pt Thread-Index: Ac9CrcIZpAeApXLaR8CwvETVzMByzA== [/PHP] I've been fighting against this and i already turn off "Require RFC-compliant HELO" and even "Require HELO before MAIL" but nothing seems to work. I disabled DKIM, SPF, enabled again, and nothing works. How can I solve this? My clients are angry (I understand that) and there's nothing more I can do by now. The problem is not only on Outlook (yes, i tried "my server requires authentication"). Even when I try to send a email to those specific webmails by Cpanel software I receive that error. Can you help me?