Here's exactly what one of the messages said (and yes I did contact our ISP but they were unable to help me):

Your message did not reach some or all of the intended recipients.

 

      Subject:     regarding my test

      Sent:  2022-01-01 6:00 PM

 

The following recipient(s) cannot be reached:

 

      'Jeanie Willis' on 2022-01-01 6:00 PM

      Simon Fogarty on 2022-01-01 6:00 PM

            This message could not be sent. The client operation failed. Try sending the message again later, or contact your network administrator.

 

__________________________________________________

 

Diagnostic information for administrators:

__________________________________________________

 

Error is [0x80004005-0x00000b22-0x00000501].

 

Exchange response headers:

      request-id: 88877b6b-6d4b-726f-e73b-529338b24a78

      X-ServerApplication: Exchange/15.20.4844.015

      X-FEServer: YT1PR01CA0082

      X-BEServer: PH0PR03MB6250

      X-CalculatedBETarget: PH0PR03MB6250.NAMPRD03.PROD.OUTLOOK.COM

      X-RequestId: {A1334D13-0E71-4E21-AF12-4A48221685C4}:341

      X-ClientInfo: {2D92AED4-F7A7-4D2A-86DF-AD0F2B9B1FEF}:29240018

      X-ElapsedTime: 515

      X-BackEndHttpStatus: 200

      X-ResponseCode: 0

      X-DiagInfo: PH0PR03MB6250

      X-RequestType: Execute

__________________________________________________

 

ROPs Summary:

 

      0: ropSetProps (10) Processed(1) Completed(0)

            ROP result: 0

            Response codes: 0

      1: ropSetProps (10) Processed(1) Completed(0)

            ROP result: 0

            Response codes: 0

      2: ropFlushRecipients (14) Processed(1) Completed(0)

            ROP result: 0

            Response codes: 0

      3: ropSetProps (10) Processed(1) Completed(0)

            ROP result: 0

            Response codes: 0

      4: ropTransportSend (74) Processed(1) Completed(0)

            ROP result: 0

            Response codes: 2850

__________________________________________________

 

Response Exceptions:

 

ROP Index: 4

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.PropertyBagContext.ExecuteCallback(Action operation)

   at Microsoft.Exchange.Data.EventHandler.EventDispatcher.Dispatch[TEvent](TEvent currentEvent)

   at Microsoft.Exchange.Data.Storage.CoreItem.PreprocessItem()

   at Microsoft.Exchange.Data.Storage.CoreItem.TransportSend(PropertyDefinition[]& properties, Object[]& values)

   at Microsoft.Exchange.Data.StoreObjects.CoreItemMetricLogger.TransportSend(PropertyDefinition[]& properties, Object[]& values)

   at Microsoft.Exchange.RpcClientAccess.Handler.Message.TransportSend()

   at Microsoft.Exchange.RpcClientAccess.Handler.RopHandler.<>c__DisplayClass156_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(14), failure enum(13), HResult(0x80004005), EC(2850).