I am having an issue where users at our parent company are sending emails with attachments to multiple users withing our Exchange environment and some of the messages have the attachment and others don't. I view the logs on our spam filter and I see the users who do get the message include the correct .pdf extension in the body of the mail. Users who aren't getting the message have the attachment listed as winmail.dat. To further complicate things sometimes a user from the parent company will send a message in rich text and the attachment will not go through then they will send it a second time and it will go through.
It seems like HTML format works all the time but I'm not sure why a format issue would suddenly pop up when everything was working correctly before. I can send emails from Google in both rich text and html and they come through no problem.
The emails that show up with the winmail.dat extension in the email header have a "extra line breaks in this message were removed" at the top of the Outlook message. The parent company is on the only sender we are having this issue with.