A subset of users on Outlook using MAPI are plagued with password prompts.  At first users were being prompted to enter their 365 credentials (using Outlook 365) at least daily, some as often as every few hours, and their Smartermail passwords at least daily.  So, we gave the client the registry patch to force autodiscover directly to the Smartermail server-- putting only the Smartermail server under HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover\RedirectServers.

This stopped the MS 365 login prompts, presumably because Outlook is now communicating directly with the Smartermail server instead of proxying via 365 cloud.

However, the Smartermail Password prompts have NOT stopped.  Some users are still getting prompted for their email password every day or even every few hours.  

IT team has cleaned out Windows Credential Manger of any email-related accounts.  They've also deleted the Outlook profile and created a new one for affected users.  Still the password prompts continue, and these users are complaining to executives, which in turn is raining back down on us.

Anyone seen this?  Any ideas?  Fixes?  Workarounds?  SM version 8930 currently but it's been happening across multiple prior versions, too.  

NOTE:  Checking the "Remember my credentials" box is ineffective.