Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Thanks for the reply but I had already tried both of those things but could find no duplicate value.
For anyone else with the same issue it was because the account had been deleted from AD but a sync had not taken place. In the end I had to restore the deleted version of the account using AD recycle bin (renamed the new account first), force a sync, delete the account again, force a sync and then I could rename the new account back to the correct name and use the SMTP attribute