See this KB article.

Thanks JohnnyQuest - Unfortunately, our maintenance contract expired recently and I can't get to that KB. 

Mmmm... so you have no support for the product either.  Not a great place to be when working on a high profile project?

Here's the meat of KB for you.

Cause:

This happens due to Microsoft Calendar Repairing Assistant sending these notification because it considers these meetings / appointments conflicting.

Relevant resolution:  (may require some tweaks as this is old information)

Thanks JohnnyQuest - We are on Exchange on-premises but I'll give this a shot. Does the KB mention whether the culprit is the sender or receiver mailbox or the source or destination Exchange servers?

You need set this on both sides:

For all mailboxes on Exchange 2013:

Get-Mailbox -ResultSize unlimited -Filter {CalendarRepairDisabled -eq $false} | Set-Mailbox -CalendarRepairDisabled $true

For Exchange 2013 on per mailbox basis:

Set-Mailbox -Identity <mailbox@yourdomain.com> -CalendarRepairDisabled $true

The cmdlet syntax should be pretty similar for newer versions of Exchange.

Thank you! I'll give it a try.