I notice you say 'Replies'. So this issue is not occuring in a new Write mesage. When you read an email and click on 'Reply' to correspond, the entirety of the original email is copied into the 'Write' window. This means everything is copied including any css html that was originally included in the original email. If that original email contained css html to force double spacing then it will influence what you are writing as the reply. This occurs more frequently when the original sender used eg: a Microsoft product to create the email. They may have used Outllok email client or more likely copy pasted something from eg: MSWord. That would have copy pasted a load of css stuff. It's not helpful to the recipient.

You could try this as a test: Select email 'View' > 'Message Body as' and select 'Plain text' Now click on 'Reply' to open a new Write message and see if you have more control.

Thank you for the quick and accurate reply. The simple fix does stop the reply emails from double spacing with the enter key.