Credential prompt when accessing on-premises public folders

Symptoms

Consider the following scenario:

• You have a hybrid environment of Microsoft Exchange Server on-premises and Microsoft Exchange Online.
• You create public folders in Exchange Server on-premises.

In this scenario, users in Exchange Online are repeatedly prompted for credentials when they try to access on-premises public folders.

Additionally, the Exchange Server log (ExchangeInstallDirectory\Logging\MAPI Client Access) contains the following error entry:

Can't act as owner of a MailUser object.

Here's an example of the error message details:

"RpcDispatch: [LoginPermException] 'User SID: <SID number>' can't act as owner of a MailUser object '/o=ExchangeLabs/ou=Exchange Administrative Group (<Group name>)/cn=Recipients/cn=<User identity>-OnPrem' with : <SID number> and MasterAccountSid (StoreError=LoginPerm) at M.E.R.Server.UserManager.User.CorrelateIdentityWithLegacyDN(ClientSecurityContext clientSecurityContext) at M.E.R.Server.RpcDispatch.<>c__DisplayClassc.<Connect>b__8() at M.E.R.Server.RpcDispatch.ExecuteWrapper(Func`1 getExecuteParameters, Func`1 executeDelegate, Action`1"

Cause

In a hybrid environment, Exchange Online user mailboxes are associated with the owners' on-premises mail user accounts. This issue occurs if the passwords of the on-premises mail user accounts are expired.

Resolution

To resolve this issue, reset the passwords of the on-premises user accounts that are associated with the Exchange Online mailboxes.