You can use examples like:
76671

76446

76208

For additional answers for this question, however this error is entirely out of our remit. You can use the below response as a copy pasta:

"Thank you for your email today. 


These errors are generated by the domain that sends the invites. This is a known issue with Microsoft when migrating from Exchange 2010 to Exchange 2016, and is sadly out of EMS control, or our remit to assist. 

All I can really recommend for this situation, is that the remote domain generating the invites needs to contact Microsoft to get support for this. 

Apologies I cannot be more helpful [Customer Name], but please do let me know if you have any questions. "

This information is based off of:
https://answers.microsoft.com/en-us/outlook_com/forum/all/not-supported-calendar-messageics-after-migration/76b2f7b7-718b-48ef-8cb9-a0bb278b8a17

This is a known issue for Microsoft migrations, and happens outside of EMS.