I am pretty sure that the message 550 Request failed; Mailbox unavailable is the response from the remote/destination server.
I tried the calendar app now on my DEV playground and successfully sent an invitation and received it as well. I did notice though, that the sent email had a “Reply-To” Header in it. Some providers don’t like these and block these in their mail servers. In that case the mail server only sends the response 550 Request failed; Mailbox unavailable which is quite vague.
Could you try and send an email straight from your mail server to the recipients and see if they get your mails? If that’s the case, there needs to be a solution to remove the “Reply-To” Header from the mails of NC.