can you put in place a policy to stop users using the new outlook?

You can prevent it with a flag in O365. Let me see if I have that page open still.

@Dennis Parker thanks

https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/outlook-on-the-web/enable-disable-employee-access-new-outlook Set-CASMailbox -Identity <MailboxIdentity> -OneWinNativeOutlookEnabled <$true | $false>

@Dennis Parker your a star

Just been heavily on my todo list the past few days, but thank you. 🙂

haha 🙂