Exchange: Delivery has failed to these recipients or distribution lists – IMCEAMAILTO

Clicked a link to email somebody outside the organisation, Outlook 2007 opened a new mail window as you’d expect. Typed the message, clicked Send, and immediately got a message back from Exchange (2007):

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Delivery has failed to these recipients or distribution lists:

‘firstname.lastname@hp.com’
A problem occurred during the delivery of this message. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message later, or provide the following diagnostic text to your system administrator.
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Sent by Microsoft Exchange Server 2007
Diagnostic information for administrators:

Generating server:hubtransportserver.active.directory.mydomain.co.uk
IMCEAMAILTO-firstname+2Elastname+40hp+2Ecom@mydomain.co.uk
#550 5.4.4 ROUTING.NoNextHop; unable to route ##
Original message headers:
Received: from mailboxserver.active.directory.mydomain.co.uk ([zzzz::zzzz:zzzz:zzz:zzzz]) by  hubtransportserver.active.directory.mydomain.co.uk ([192.168.251.45]) with mapi; Thu, 28 Oct  2010 11:09:40 +0100
Content-Type: application/ms-tnef; name=”winmail.dat”
Content-Transfer-Encoding: binary
From: Me me@mydomain.co.uk
To: “‘firstname.lastname@hp.com'” IMCEAMAILTO-firstname+2Elastname+40hp+2Ecom@mydomain.co.uk
Date: Thu, 28 Oct 2010 11:09:39 +0100
Subject: Something Amazing
Thread-Topic: Something Amazing
Thread-Index: xxxxxxxxxxxxxxxxxxxxxxxx
Message-ID: yyyyyyyyyyyyyyyyyyyyyyyyy@mailboxserver.active.directory.mydomain.co.uk
Accept-Language: en-US, en-GB
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: yyyyyyyyyyyyyyyyyyyyyyyyy@mailboxserver.active.directory.mydomain.co.uk
MIME-Version: 1.0

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…which was nice.

Tried to send the message again, this time by typing the email address into the To box manually, same error message back from Exchange.

Opened a new email, typed the address into the To field, double-clicked the address once Outlook had underlined it and checked the E-Mail address field – it looked fine, correctly contained the email address that I was trying to send to.

I then tried this to a different external email address that I know I can send mail to and compared the two E-mail Properties boxes:

Not working:

Working:

Spot the (important) difference? On the properties for the address that doesn’t work the E-mail type field is set to MAILTO, whereas it should be set to SMTP and greyed out, as in the bottom picture. Clicking the Internet Type button corrects this, and then the message will send fine.

This is ultimately caused because the original link I clicked had a malformed mailto URL on the email address, which I didn’t notice at the time, as per here.