I had the same issue, I made the correction below and it works
1. From the Domino Administrator, click the Configuration tab and expand the Messaging section.
2. Click Configurations.
3. Select the Configuration Settings document for the mail server or servers you want to administer, and click Edit Configuration.
4. Click the MIME -> Conversion Options -> Outbound tab.
5. Complete the following fields and then click Save & Close: Message Content to "Convert from Notes to Plain Text and HTML"