An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

[MapiExceptionUnknownUser: MapiExceptionUnknownUser: Unable to open message store. (hr=0x80004005, ec=1003)
Diagnostic context:
    Lid: 55847   EMSMDBPOOL.EcPoolSessionDoRpc called [length=630]
    Lid: 43559   EMSMDBPOOL.EcPoolSessionDoRpc returned [ec=0x0][length=240][latency=15]
    Lid: 52176   ClientVersion: 15.0.516.30
    Lid: 50032   ServerVersion: 15.0.516.6030
    Lid: 23226   --- ROP Parse Start ---
    Lid: 27962   ROP: ropLogon [254]
    Lid: 17082   ROP Error: 0x3EB     
    Lid: 26937  
    Lid: 21921   StoreEc: 0x3EB     
    Lid: 27962   ROP: ropExtendedError [250]
    Lid: 1494    ---- Remote Context Beg ----
    Lid: 61208   StoreEc: 0x8004010F
    Lid: 57721   StoreEc: 0x8004010F
    Lid: 47536  
    Lid: 64952   StoreEc: 0x3EB     
    Lid: 56872   dwParam: 0xFE
    Lid: 42712   StoreEc: 0x3EB     
    Lid: 10786   dwParam: 0x0        Msg: 15.00.0516.025:server
    Lid: 1750    ---- Remote Context End ----
    Lid: 26849  
    Lid: 21817   ROP Failure: 0x3EB     
    Lid: 26297  
    Lid: 16585   StoreEc: 0x3EB     
    Lid: 32441  
    Lid: 1706    StoreEc: 0x3EB     
    Lid: 24761  
    Lid: 20665   StoreEc: 0x3EB     
    Lid: 25785  
    Lid: 29881   StoreEc: 0x3EB     ]
   Microsoft.Mapi.MapiExceptionHelper.InternalThrowIfErrorOrWarning(String message, Int32 hresult, Boolean allowWarnings, Int32 ec, DiagnosticContext diagCtx, Exception innerException) +61
   Microsoft.Mapi.MapiExceptionHelper.ThrowIfError(String message, Int32 hresult, IExInterface iUnknown, Exception innerException) +91
   Microsoft.Mapi.ExRpcConnection.OpenMsgStore(OpenStoreFlag storeFlags, String mailboxDn, Guid mailboxGuid, Guid mdbGuid, String& correctServerDn, ClientIdentityInfo clientIdentityAs, String userDnAs, String applicationId, Byte[] tenantHint, CultureInfo cultureInfo) +1683
   Microsoft.Mapi.MapiStore.OpenMapiStore(String serverDn, String userDn, String mailboxDn, Guid guidMailbox, Guid guidMdb, String userName, String domainName, String password, String httpProxyServerName, ConnectFlag connectFlags, OpenStoreFlag storeFlags, CultureInfo cultureInfo, Boolean wantRedirect, String& correctServerDN, ClientIdentityInfo clientIdentity, String applicationId, Client xropClient, Boolean wantWebServices, Byte[] clientSessionInfo, TimeSpan connectionTimeout, Byte[] tenantHint) +2105
   Microsoft.Exchange.Data.Storage.MailboxSession.ForceOpen(MapiStore linkedStore) +23073

[MailboxUnavailableException: Cannot open mailbox /o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=4bb7cd002ef7410692b92adfd72e85fc-Admin.]
   Microsoft.Exchange.Data.Storage.MailboxSession.ForceOpen(MapiStore linkedStore) +51774
   Microsoft.Exchange.Data.Storage.MailboxSession.Initialize(MapiStore linkedStore, LogonType logonType, ExchangePrincipal owner, DelegateLogonUser delegateUser, Object identity, OpenMailboxSessionFlags flags, GenericIdentity auxiliaryIdentity) +4354
   Microsoft.Exchange.Data.Storage.<>c__DisplayClass16.<CreateMailboxSession>b__14(MailboxSession mailboxSession) +156
   Microsoft.Exchange.Data.Storage.MailboxSession.InternalCreateMailboxSession(LogonType logonType, ExchangePrincipal owner, DelegateLogonUser delegatedUser, CultureInfo cultureInfo, String clientInfoString, IBudget budget, Action`1 initializeMailboxSession, InitializeMailboxSessionFailure initializeMailboxSessionFailure) +2333
   Microsoft.Exchange.Data.Storage.MailboxSession.CreateMailboxSession(LogonType logonType, ExchangePrincipal owner, DelegateLogonUser delegateUser, Object identity, OpenMailboxSessionFlags flags, CultureInfo cultureInfo, String clientInfoString, PropertyDefinition[] mailboxProperties, IList`1 foldersToInit, GenericIdentity auxiliaryIdentity, IBudget budget) +1022
   Microsoft.Exchange.Data.Storage.MailboxSession.ConfigurableOpen(ExchangePrincipal mailbox, MailboxAccessInfo accessInfo, CultureInfo cultureInfo, String clientInfoString, LogonType logonType, PropertyDefinition[] mailboxProperties, InitializationFlags initFlags, IList`1 foldersToInit, IBudget budget) +1198
   Microsoft.Exchange.Data.Storage.MailboxSession.Open(ExchangePrincipal mailboxOwner, ClientSecurityContext clientSecurityContext, CultureInfo cultureInfo, String clientInfoString) +188
   Microsoft.Exchange.Clients.Owa2.Server.Core.OwaClientSecurityContextIdentity.CreateMailboxSession(ExchangePrincipal exchangePrincipal, CultureInfo cultureInfo) +533
   Microsoft.Exchange.Clients.Owa2.Server.Core.RequestDispatcher.HandleLanguagePost(RequestContext requestContext, CultureInfo culture, String timeZoneKeyName, Boolean isOptimized, String destination) +647
   Microsoft.Exchange.Clients.Owa2.Server.Core.RequestDispatcher.DispatchIfLanguagePost(RequestContext requestContext) +642
   Microsoft.Exchange.Clients.Owa2.Server.Core.RequestDispatcher.InternalDispatchRequest(RequestContext requestContext) +620
   Microsoft.Exchange.Clients.Owa2.Server.Core.RequestDispatcher.DispatchRequest(RequestContext requestContext) +297
   Microsoft.Exchange.Clients.Owa2.Server.Core.OwaRequestHandler.OnPostAuthorizeRequest(Object sender, EventArgs e) +352
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +80
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +165