In Control Panel, click User Accounts > Credential Manager > Windows Credentials.To work around this issue, remove the cached credentials from Credential Manager on the users' computers. This issue occurs because Outlook 2016 or 2010 keeps trying to use the credentials that were cached for the old RPC over HTTP protocol. After the change, Outlook 2016 or Outlook 2010 users are repeatedly prompted for credentials to connect to Exchange Server.
You change the protocol that your Outlook clients use to access Microsoft Exchange Server from RPC over HTTP to MAPI over HTTP.
Outlook repeatedly prompts for credentials when using MAPI over HTTP to connect to Exchange
