Wanted to give an update on this and what we discovered. We have first found out that there are a lot of reasons this error message can pop up, so this fix may not work for everyone.
What we discovered with our design clients that when they had over roughly 900 fonts activated on their computer, Office would throw up these errors, and Outlook would have trouble sending emails with attachments. When we disabled fonts or activated less
than 900 (rough number was ~935 when I last checked), Office acted fine.
We have been working with Microsoft development on this issue, but no resolution is in sight. For now, we are having our users use Office 2011 for the time being and no issues there, but that won't be a permanent solution since 2011 will likely not be supported
in High Sierra later this year, so I hope Microsoft gets it fixed.