External Outlook Items getting added as Tentative automatically

Possibly take action if message is from outside the organisation and x-ms-exchange-messagesentrepresentingtype header matches 1 or 2?

Definitely try that with a non-intrusive action to begin with. Outlook meeting objects are slippery critters at the best of times, and my suggested directionality test might be no good. If so, possibly you could restate the predicate if your list of accepted domains or hybrid arrangements are not too complex?