We've seen this several times and haven't been able to find a cause nor a fix.

On occasion, an HTML email that we create, the preview text (whether it's rendered from the first text), or by a hacktastic hidden div of preview text, it will sometimes show up with no spaces between words in Both Outlook for Mac and the Office 365 web client. Any idea what causes this and how to fix and/or avoid it?

Preview shows no spaces between words