When you install the latest version of SmarterMail, your server needs time to update the UID's for all users and emails on the server. This can take some time especially on larger installations.
If you have been running the latest version for days or weeks and a customer is having an issue, we suggest the user remove their account in Outlook and re-add it to see if it resolves the problem.
If you are still having issues, then I would suggest opening a ticket with our support department and we can help evaluate the issue.
This modification of UID's has been done on millions of mailboxes and we know the process is working properly and the changes by Outlook 2016 have been accommodated. We have yet to see a server that had an issue or did not fully convert all UID's but there is always a first. If you continue to have issues, please contact our support department. As always, if we discover there is a BUG, we refund the ticket. Here to help!