If you are using Outlook for Windows, you may experience the following issues.
・ An attached file is missing when the message is received externally.
・ An attached file is replaced by Winmail.dat when the message is received externally.
・ You are sending e-mail with no attachment, but the message is received with Winmail.dat.
Missing attachments can be caused by several scenarios, but when you see Winmail.dat, there is no doubt that these issues are caused by TNEF.
TNEF is Transport Neutral Encapsulation Format, used by Outlook for Windows and Exchange Server to make things work for Outlook specific features like Voting Buttons.
You need to be aware that TNEF is used in various conditions, not only when you select Rich Text Format for a message in Outlook.
When TNEF is used, the additional data to a message body (including attachment files) are converted into Winmail.dat.
And if recipients are using a mail client which does not convert Winmail.dat back into readable-form, it will just show up as an unviewable attachment file.
It is difficult to guess whether or not TNEF will be used for a message since recipient configurations and various elements of a message have to be taken into account, and sometime TNEF is used for complex reasons.
So, it is difficult (almost impossible) for users to completely avoid using TNEF in their operations.
If you want to avoid these issues caused by TNEF, the following methods can be helpful.
When you are using Outlook with Exchange account
When you send a message from an Exchange account, the decision to use TNEF is made by Exchange Server, not by Outlook.
So, in order to disable TNEF in this situation, you need to configure Exchange Server.
To be more specific, please set TNEFEnabled to false so TNEF is never used for those domain/domains.
Please refer to the articles below to know more about Remote domains.
Title : Remote domains in Exchange Online
Title : Remote domains (Exchange Server 2013)
Title : Configure Remote Domain Properties (Exchange Server 2010)
URL : https://technet.microsoft.com/en-us/library/bb124931.aspx
When you are using Outlook with POP/IMAP/EAS account
With setting the registry below, you can avoid using TNEF.
Note that you cannot use this method with an Exchange account.
Path : HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\Preferences
Name : DisableTNEF
Type : REG_DWORD
Data : 1
Note xx.0 in this registry path corresponds to the Outlook version (16.0 = Outlook 2016, 15.0 = Outlook 2013, 14.0 = Outlook 2010).
Limitation of Outlook when you prohibit TNEF
When you prohibit using TNEF, you cannot use the following Outlook features.
・ You cannot send messages with custom Forms. The scripts and properties will be removed.
・ iCalendar is always used for meeting requests.
・ You cannot use the Task Request feature. A task request message will be replaced by a normal message.
・ You cannot use Voting Buttons. You can set it before sending, but recipients may not see any buttons.
・ You cannot use embedded OLE (Object Linking and Embedding) for a message body. These are replaced by pictures.
and so on.