We recently started using SendGrid as an outbound smarthost for our Exchange environment after switching to an inbound security provider that did not have outbound messaging available. For the most part, SendGrid has been fantastic but we recently noticed a small issue with Outlook calendar meeting requests being delivered to external parties. While we are sending the requests in the same manner as always, recipients are now receiving plain emails with "not supported calendar message.ics" attachments. They are able to open the attachment to add it to their calendars but it is no longer received as a native meeting request.

I already tried disabling the Footer app in case that was causing some issue but it had no affect.