We get lots of calls from email clients who can not open attachments sent to them usually it is a .dat file

When someone sends an attachment using “Rich Text” formatting in Microsoft Outlook the inserted attachments become .dat file which can only be opened if the recipient is also using Outlook and sometimes only if they are in an Exchange environment.

The sender is using Microsoft Outlook.

Recipients receive a winmail.dat attachment
See: https://support.office.com/en-US/article/Recipients-receive-a-winmaildat-attachment-1735ba97-39b8-40d4-ba17-0e0150ef87a8

Recipients notify you that the e-mail message you sent appears to them as a message with an attachment called winmail.dat.
Sender is using the Rich Text message format, which the recipient’s e-mail program cannot interpret correctly.
Resolution: (Send this to the sender)
Send your messages in either HTML or plain text format.
Send messages in HTML or plain text format

1. On the Tools menu, click Options, and then click Mail Format.what is a dat file

2. Under Message Format, in the Compose in this message format list, click HTML or Plain Text.

3. Click OK.