I get a message telling me that there isn't an email address associated with the Calendar event when trying to invite people, The calendar invite gets stuck in my outbox.  But, I can send regular emails just fine. 

Does the target email address is exist currently? type the address manually and invite the certain address again.

Make sure you are using Exchange account and please invite a email address inside your organization to check the result.

Thanks.

Tony Chen
TechNet Community Support