Outlook + TNEF, the reverse problem: outlook doesn't understand winmail.dat

Question

A lot of information can be found about people with other mail clients not being able to read TNEF encoded (winmail.dat) messages, but I am having the opposite problem: *Outlook* can not read them anymore in our organization.
Mail servers are all Exchange 2010, Outlook versions 2010 and 2013.
Internal mails in TNEF/RTF format are fine.
In external mails coming in from the internet, if they arrive in TNEF, Outlook shows the plain ASCII text and the winmail.dat attachment.
The only thing I can find that may cause it, is a header inserted into the mail by our firewall. Now my question is, can that really be the reason?
Example snippet from an incoming mail with winmail.dat attachment, the "X-Watchguard-Antivirus" header is what I'm talking about.
--_000_B083931C9A44A84C8FF71C48801A6D5FACE1D820SRVMAIL1KSCCJHL_
Content-Dis; filename="winmail.dat"
Content-Transfer-Encoding: base64
Content-Type: application/ms-tnef; name="winmail.dat"
X-WatchGuard-AntiVirus: scanned 'winmail.dat'. clean action=allow
eJ8+IgAkAQaQCAAEAAAAAAABAAEAAQeQBgAIAAAA5AQAAAAAAADoAAEJgAEAIQAAAEEzQUExN0Y4
NDMwOEU3NDdCNkMxMzIyODkzRTRBRTg1ACUHAQ2ABAACAAAAAgACAAEFgAMADgAAAN8HAQAOAA0A
The "Content-disposition" line somehow got mangled when I quoted it above, the real line reads correctly "Content-Disposition colon attachment semicolon filename=...".

Answer 1

Hi,
The common reason that recipients receive emails as winmail.dat is that they are not using Outlook/Exchange, their email clients can't decipher the RTF/TNEF format thus the email is shown as winmail.dat.
In your scenario, you are using Outlook and Exchange, you should receive RTF emails correctly - It's proved with the Internal emails.
I can't be sure whether it's caused by the firewall, but if all external emails are facing this issue, it's most likely caused by the firewall.
To verify this, is it possible for you to configure the firewall not to insert the header? If it makes a difference to external emails that they can be read then, we can conclude the firewall was the culprit.
Regards,
Melon Chen
Forum Support
Come back and mark the replies as answers if they help and unmark them if they provide no help.
If you have any feedback on our support, please click
here