We are aware of this issue and a bug# 4146719 has been logged for the same.
The workaround at the moment is to write the content on a notepad and then paste it in the message field.
That's exactly what I did as workaround.
We have released a patch today that fixes the issue about spaces/line feed not working in Send and Track. If you are not already updated to the latest patch, simply open Acrobat and visit Help > Check for updates to apply this patch.
More details about this release and bug fixes is available here: https://helpx.adobe.com/acrobat/release-note/acrobat-dc-june-02-2016.html
Please give it a try and let us know.