Are you getting winmail.dat attachments in your email?
The quick reason why you are getting those attachments is because the person sending you the attachment is using Microsoft Outlook as their email client. The sender can still use Outlook but will need to make a couple of modifications on their side before sending you an attachment.
First they will need to make sure that their message is formatted in Plain Text or HTML. In a new message window, they will need to click on Options tab and then select Plain Text or HTML from the Format section.
Next they will need to make sure that they enter in your full email address in the To or CC field (ex: firstname.lastname@example.org) instead of just your username and letting it auto-correct from the Global Address List.
Doing these steps should correct the problem you are seeing. As always, please contact the Helpesk if you need assistance in this matter.