Emails refused

Hello There,

I configured your plugin on a client site. He is sending out emails with a gmail address (No custom domain for emails) but some recipients cannot be reached.

I receive notification of emails being refused by some providers.

Details follow:

Delivery to the following recipient failed permanently:

[email protected]

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain fastwebnet.it by mx1.fastwebnet.it. [85.18.95.16].

The error that the other server returned was:
554 Message refused

—– Original message —–

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=gmail.com; s=20120113;
h=from:sender:mime-version:date:message-id:subject:to:content-type;
bh=uX9DeUMW99bBkvDMt1F9iSWPRo4C+U2ky9ZgkyZMAe0=;
b=fImTu2qoX2XmUr/2t9KZvNMVhrnz64z/DhnyTjTShIF+USHwN2YS36EMeXT66C93j6
4yiTps8wAuRbFqOZpxspCs6vSjs3Ap3fehTHFxHiqyRMEeLvmIRLCOnMXY+hOqMbcTGG
lWxeFeBVyYya6K2RRhfqq2tYnebaAAHXu75JXeaFU7AD4GcN4/53qjmA9/nUDhq4fMcr
pQiu5kyyb/XJTqTKPG3RpZUmkozB9g3WsLhkrBlsSOt/qcRjk44E9P2JIhvAGpc+ZYct
Lds8HnJbyb6Gtgs5/ZwK9D5n7uMTLs0nvPoZyRvESgGoo0r4GEaEdskc4C0wTGBtwFON
Fp+g==
X-Received: by 10.194.88.102 with SMTP id bf6mr48840237wjb.129.1450180321065;
Tue, 15 Dec 2015 03:52:01 -0800 (PST)
Received: from 145797634783-6f727cnga4g27kbu4cegmdc13s2a55b8.apps.googleusercontent.com
named unknown by gmailapi.google.com with HTTPREST; Tue, 15 Dec 2015 03:52:00 -0800
X-Mailer: Postman SMTP 1.7.2 for WordPress (https://ww.wp.xz.cn/plugins/postman-smtp/)
From: Neurologia Roma – Polisonnografia <[email protected]>
Sender: [email protected]
MIME-Version: 1.0
Date: Tue, 15 Dec 2015 03:52:00 -0800
X-Google-Sender-Auth: JhuY9BFBFS-AytvcgLK0w-sYi38
Message-ID: <CAAbiufwQ33QDgHKe7a2TEguX5GJtm0VAzRpAHk1U6khCqZdv8g@mail.gmail.com>
Subject: La conferma del suo appuntamento con Polisonnografia.com.
To: [email protected]
Content-Type: multipart/alternative; boundary=047d7bf19872e5c6110526ee6ab5