appears to be a spam filter/block, I assume on the receiving account?
Response Exceptions:
ROP Index: 5
ROP Associated: ropTransportSend (74)
Microsoft.Exchange.Data.Storage.OutboundSpamException: WASCL UserAction verdict is not None. Actual verdict is Suspend, ShowTierUpgrade.
at Microsoft.Exchange.Data.Storage.WasclHelper.PerformAntiSpamCheck(IStoreSession session, ICoreItem coreItem, Func`1 activityScopeGetter)
at Microsoft.Exchange.Data.Storage.Adapters.ObjectEventHandlers.WasclEventsHandler.Handle(BeforeItemSendEvent beforeItemSendEvent)
at Microsoft.Exchange.Data.StoreObjects.Extensibility.InlineEvent.InlineEventHandlerAdapter`1.InternalHandle(TEvent eventObject)
at Microsoft.Exchange.Data.StoreObjects.PropertyBagContext.ExecuteCallback[T](Func`1 operation)
at Microsoft.Exchange.Data.EventHandler.EventDispatcher.<>c__DisplayClass6_3`1.<Dispatch>b__0()
at Microsoft.M365.Core.Diagnostics.LatencyMeasurementHelper.ExecuteWithLatencyMeasure(Action action, Int64& latency)
at Microsoft.Exchange.Data.EventHandler.EventDispatcher.Dispatch[TEvent](Func`3 eventFactory, IEventResponseHandler eventResponseHandler, EventContext eventFilter)
at Microsoft.Exchange.Data.Storage.CoreItem.<PreprocessItem>b__98_0()
at Microsoft.Exchange.Data.Storage.Performance.MailboxOperationPerformanceTracker.ExecuteWithLatencyTracking(ActivityOperationType operation, Action action, String instance)
at Microsoft.Exchange.Data.Storage.CoreItem.TransportSend(PropertyDefinition[]& properties, Object[]& values)
at Microsoft.Exchange.RpcClientAccess.Handler.Message.TransportSend()
at Microsoft.Exchange.RpcClientAccess.Handler.RopHandler.<>c__DisplayClass154_0.<TransportSend>b__0()
at Microsoft.Exchange.RpcClientAccess.Handler.ExceptionTranslator.TryExecuteCatchAndTranslateExceptions[TResult](Func`1 protectedCode, Func`2 errorCodeExtractor, Boolean noThrow, TResult& result, Exception& exception, ErrorCode& errorCode)
Transport-Send failed: failure enum(25), HResult(0x00000000), EC(2850).
Transport-Send failed: failure enum(22), HResult(0x00000000), EC(2850).
Submit-Message failed: message id(2), failure enum(13), HResult(0x80004005), EC(2850).