Transport Neutral Encapsulation Format (TNEF-encoded, winmail.dat or win.dat) messages aren't processed by the instance

Issue

Problem

Transport Neutral Encapsulation Format or TNEF is a proprietary email attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most often named winmail.dat or win.dat, and has a MIME type of Application/MS-TNEF.

These files are not understood by the instance.

Symptoms

You will recognise this problem because:

Emails from some Microsoft clients or using Exchange server do not get processed.
Incoming emails from those clients contains a winmail.dat file attached
The body of the email does not contains the information required.

Cause

Processing of rich formatting or TNEF-encoded messages is not available at this time.

When customers send TNEF-encoded messages to technical support, it includes two parts:
TNEF-encoded, base64 strings which contain rich-text that Outlook understands but most mail clients don't (also images and attachments)
Plain text versions of the text.

Often, these messages include an attachment named "winmail.dat".

Resolution

Customers can disable TNEF on their Exchange server for messages sent to ServiceNow, thereby forcing the messages to send in MIME-types encoding instead.

The exchange setting that can be enabled per-domain to resolve this:
http://technet.microsoft.com/en-us/library/aa997857.aspx

TNEFEnabled:

The TNEFEnabled parameter specifies whether Transport Neutral Encapsulation Format (TNEF) message encoding is used on messages sent to the remote domain. Valid values for this parameter are $true, $false, or $null. The action associated with each value is as follows:
$true TNEF encoding is used on all messages sent to the remote domain.
$false TNEF encoding isn't used on any messages sent to the remote domain.
$null TNEF encoding isn't specified for the remote domain. TNEF encoding for recipients in the remote domain may be specified by the following:
Value of the UseMapiRichTextFormat parameter for any mail user or mail contact objects
Sender's per-recipient settings in Microsoft Office Outlook
Sender's default Internet message settings in Outlook
The default value is $null

It should be possible to set this to $false for "service-now" in the Exchange server, which should force the message to be sent with a plain text portion and an HTML portion, both of which should be readable by the system. Your exchange admin can consider trying this option for the service-now domain.