Workfow fails after re-assigning tasks to individuals on CRM more than once

Hello everyone,

I have created a CRM workflow that sends an email to the person who gets a task assigned to him. when I first assign the task to someone the e-mail works and gets sent to the assigned person.

but when when the person whose assigned the task try to re-assign the task to someone else the email failed to send. below are error logs from settings\system jobs\workprocess 

I'd appreciate any help

Here's my workflow snapshots:

Error Messages:

This message cannot be sent to all selected recipients. Verify that both the sender and the recipients have valid e-mail addresses, and that the recipients are allowed to receive e-mails

Plugin Trace:

[Microsoft.Xrm.Sdk.Workflow: Microsoft.Xrm.Sdk.Workflow.Activities.SendEmail]
[SendEmailStep1]

Error Message:

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: Object address not found on party or party is marked as non-emailableDetail: 
<OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts">
  <ErrorCode>-2147218688</ErrorCode>
  <ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
  <Message>Object address not found on party or party is marked as non-emailable</Message>
  <Timestamp>2013-06-10T09:02:51.9174617Z</Timestamp>
  <InnerFault>
    <ErrorCode>-2147218688</ErrorCode>
    <ErrorDetails xmlns:d3p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" />
    <Message>Object address not found on party or party is marked as non-emailable</Message>
    <Timestamp>2013-06-10T09:02:51.9174617Z</Timestamp>
    <InnerFault i:nil="true" />
    <TraceText i:nil="true" />
  </InnerFault>
  <TraceText>[Microsoft.Xrm.Sdk.Workflow: Microsoft.Xrm.Sdk.Workflow.Activities.SendEmail]
[SendEmailStep1]
</TraceText>
</OrganizationServiceFault>
   at Microsoft.Crm.Extensibility.OrganizationSdkServiceInternal.Execute(OrganizationRequest request, CorrelationToken correlationToken, CallerOriginToken callerOriginToken, WebServiceType serviceType)
   at Microsoft.Crm.Extensibility.InprocessServiceProxy.ExecuteCore(OrganizationRequest request)
   at Microsoft.Crm.Workflow.Services.SendEmailActivityService.<>c__DisplayClass1.<SendEmailInternal>b__0(IOrganizationService sdkService)
   at Microsoft.Crm.Workflow.Services.ActivityServiceBase.ExecuteInTransactedContext(ActivityDelegate activityDelegate)
   at Microsoft.Crm.Workflow.Services.SendEmailActivityService.ExecuteInternal(ActivityContext executionContext, SendEmail sendEmail)
   at Microsoft.Crm.Workflow.Services.SendEmailActivityService.Execute(ActivityContext executionContext, SendEmail sendEmail)
   at System.Activities.CodeActivity.InternalExecute(ActivityInstance instance, ActivityExecutor executor, BookmarkManager bookmarkManager)
   at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation)

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Mohammed JH