Emails with attachments sent to iOS or OSX devices receive winmail.dat
A very frustrating issue that I have seen at many sites are winmail.dat attachments. This issue occurs when an email with an attachment is sent from an Exchange server to an external user with an Apple iOS or OSX device. The recipient receives the email however the attachment is replaced with an unusable winmail.dat file. It is worth noting that this issue only occurs if the email is sent in rich text format, the other formats HTML and plain text are not impacted. The reason this issue occurs is due to Microsoft Exchange encoding attachments in RTF emails with the propitiatory Transport Neutral Encapsulation Format (TNEF). You can read a bit more about TNEF here.
There are some third party apps on the market that allow the iOS or OSX device to open the winmail.dat files. One example of this is Letter Opener. However this is not a practical resolution from a administrators point of view.
It is possible to prevent Microsoft Exchange from sending any RTF emails to external recipients. To resolve this issue for all external emails follow the steps below
- Open the Microsoft Exchange Management Console
- Browse through to Organization Configuration then Hub Transport
- Click on the Remote Domains tab
- Right click on the Default rule and select Properties
- Click on the Mail Format tab
- Under the Exchange rich-text format section select None
- Restart the Exchange transport service
- Open the Exchange Management Shell
- Run the following command Set–RemoteDomain –Identity * –TNEFEnabled $false
This will prevent the winmail.dat issue going forward.