I have an exchange email and just changed server hosting to a new cluster hosting with Fasthosts a few weeks ago.
I exported and re-imported the data file and all imported just fine.
However, now when sending an email, either to an existing contact, or a brand new one, it bounces back as undeliverable (about 50% of the time) but when resending the same email to the same contact by forwarding or just composing the email again it is send fine every time.
This doesn't happen on webmail and Fasthosts say its an outlook problem.
Below is an example of the error message. Any ideas as its really odd?!! Thanks!

Delivery has failed to these recipients or groups:
MY EMAIL DOMAIN
The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your helpdesk.

Diagnostic information for administrators:
Generating server: 1.exchange.profimailer.de
IMCEAEX-_o=HostedExchange_ou=Exchange+20Administrative+20G roup+20+28FYDIBOHF23SPDLT+29_cn=Recipients_cn=info 73f3341fb40@win.mail
Remote Server returned '550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found'
Original message headers:
Received: from winhexbeeu45.win.mail (10.76.18.53) by winhexbeeu37.win.mail
(10.76.18.49) with Microsoft SMTP Server (TLS) id 15.0.1178.4; Tue, 11 Jul
2017 17:45:27 +0200


Received: from winhexbeeu45.win.mail ([fe80::bd73:4003:32ee:da6a]) by
winhexbeeu45.win.mail ([fe80::bd73:4003:32ee:da6a%14]) with mapi id
15.00.1178.000; Tue, 11 Jul 2017 17:45:27 +0200
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: My Emai <info@xxxxx.com>
To: "xxxxxx@mac.com" <xxxxxx.com>
Subject:
(Ref: KD010)
Thread-Topic: EMAIL TITLE
Required (Ref: KD010)
Thread-Index: AdL6XLWFx8IA3hhORumO5nMjhC2gIw==
Importance: high
X-Priority: 1
Date: Tue, 11 Jul 2017 17:45:27 +0200
Message-ID: <2f96eb66ee1f44938b9e7fdf997280ae@winhexbeeu45.win. mail>
Accept-Language: en-GB, en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <2f96eb66ee1f44938b9e7fdf997280ae@winhexbeeu45.win. mail>
MIME-Version: 1.0
X-MS-Exchange-Transport-FromEntityHeader: Hosted
X-Originating-IP: [86.24.10.241]
Return-Path: info@MYEMAIL.com
X-1and1-Spam-Score: 0.1/10
X-1and1-Spam-Level: None
X-1and1-Expurgate-Category: clean
X-Provags-ID: V02::0IgeJTr4RGN770tvyfNdGmiwzZUpOlvQo0W5vNvmcoK0+
K4b5ObTlDZyU32wurIbBFP8EPabNVYbt9+iRrewNsIv4XYCHxf
LTQo/Oubxpg5vi1AAB0XM6c4zXZXzeOfLounqRcSv2n4LrvGcI
uNlzR4Wfeqk+79nS6jSbDykeqA7crM1yxsBinjdDH1ctXVG