Deleted Alias Still Being Used

I have an Email address in Mail (call it account 1) that I setup deliberately to use as an alias for a different account (call this account 2) that was not available on an Exchange, so I could send and receive using my account 1 but it looked like I was using account 2.

Everything worked fine on all devices - computers, phones and tablets.

Then recently, the address 2 became available as an Exchange account, so I connected to it natively and deleted the alias for account 2 that was tied to account 1.

Now, I still have account 1 and now also have account 2, but on all devices any incoming Emails to account 2 still divert into account 1 even though I have deleted the alias from account 1.

Am I missing a trick here in the settings maybe? Is there some kind of cache in Mail that can be reset maybe?

Thanks in advance.