Upgraded to windows 10 and getting errors when sending emails from Outlook. Chances are you are using Google App sync which is not yet supported on Windows 10. Google is always behind when it comes to the latest MS software updates so its always good to remember to check if Google App sync supports the upgrade before going ahead. The same is true for Outlook 2016 – currently not supported by Google App Sync.
This email message was not sent because we were unable to convert the message into Internet format (MIME). Removing attachments or removing forwarded text from the message may enable you to send the message. See http://support.google.com/a/bin/answer.py?answer=2392524 for details.
For many users the issue is resolved by running the command sfc /scannow
Open a command prompt as an administrator and run the command sfc /scannow which may well resolve the issue as it has done so for many other users.
Reference – https://productforums.google.com/forum/#!topic/apps/7enqlnpEhYQ