This is due to Outlook decrypting the file in RTF format. To avoid this issue you may change your Outlook settings by following these instructions:
- Open up your Outlook email client.
- Click on File located in the upper toolbar.
- Select the Options setting.
- From here, select the Mail category.
- Now, under Compose messages, you want to ensure the HTML or Plain Text option is selected for the Compose messages in this format.
- Next, under Message format, set When sending messages in Rich text format to Internet recipients to Convert to HTML format or Plain Test format
- Click OK to finish