E-mail not received by approvers in SharePoint native Approval Workflows

Hi,
We have a blog site in SharePoint 2007 on which the Posts List has an automated SharePoint Approval Workflow (native SharePoint Approval Workflow) configured to send an e-mail to a specific user to approve the posts whenever a new item (post) is created. Workflow related Timer Jobs are working fine (all up and running). When workflow starts, it sends an e-mail to the post creator but fails to send the e-mail to the user that approves the post (or at least e-mail is not received). We tried to change approvers but the result is the same: the email is not received.

Our analysis so far

    Emails are not sent to the Junk Email
    Looked for the SMTP configurations and they are all correct;
    We have set a SharePoint alert configured to the same user and it works: the email is successfully received by the user that approves the post (e-mail is sent and received);
    Created a new approval workflow in the same library with the same settngs as the original workflow (e-mail not received);
    Created a similar approval workflow to send an e-mail to a different user (e-mail not received);
    Created a similar approval workflow but without the “Automatically start when a new item is created" option) (e-mail not received);
    Looked for workflow related errors in SharePoint ULS Log files and found this error that is generated every time a new post is created and the workflow set to start automatically when a new item is created fires (manually started workflow do not generate errors in Log file but the emails are not received anyway);

    Error in log:

w3wp.exe (0x00C4)0x048CWindows SharePoint ServicesGeneral0UnexpectedERROR: request not found in the TrackedRequests. We might be creating and closing webs on different threads. ThreadId = 8, Free call stack =    at Microsoft.SharePoint.SPRequestManager.Release(SPRequest request)     at Microsoft.SharePoint.SPSite.Close()     at Microsoft.SharePoint.SPSite.Dispose()     at Microsoft.SharePoint.Workflow.SPWorkflowAutostartEventReceiver.AutoStartWorkflow(SPItemEventProperties properties, Boolean bCreate, Boolean bChange, AssocType atyp)     at Microsoft.SharePoint.Workflow.SPWorkflowAutostartEventReceiver.AutoStartWorkflow(SPItemEventProperties properties, Boolean bCreate, Boolean bChange)     at Microsoft.SharePoint.Workflow.SPWorkflowAutostartEventReceiver.ItemAdded(SPItemEventProperties properties)     at Microsoft.SharePoint.SPEventManager.RunItemEventReceiver(SPItemEventReceiver receiver, SPItemEventProperties properties, SPEventContext context, String receiverData)     at Microsoft.SharePoint.SPEventManager.RunItemEventReceiverHelper(Object receiver, Object properties, SPEventContext context, String receiverData)     at Microsoft.SharePoint.SPEventManager.<>c__DisplayClass8`1.<InvokeEventReceivers>b__0()     at Microsoft.SharePoint.SPSecurity.CodeToRunElevatedWrapper(Object state)     at Microsoft.SharePoint.SPSecurity.RunAsUser(SPUserToken userToken, Boolean bResetContext, WaitCallback code, Object param)     at Microsoft.SharePoint.SPSecurity.RunAsUser(SPUserToken userToken, CodeToRunElevated code)     at Microsoft.SharePoint.SPEventManager.InvokeEventReceivers[ReceiverType](SPUserToken userToken, RunEventReceiver runEventReceiver, Object receivers, Object properties, Boolean checkCancel)     at Microsoft.SharePoint.SPEventManager.InvokeEventReceivers[ReceiverType](Byte[] userTokenBytes, RunEventReceiver runEventReceiver, Object receivers, Object properties, Boolean checkCancel)     at Microsoft.SharePoint.SPEventManager.HandleEventCallback[ReceiverType,PropertiesType](Object callbackData)     at Microsoft.SharePoint.Utilities.SPThreadPool.WaitCallbackWrapper(Object state)     at System.Threading._ThreadPoolWaitCallback.WaitCallback_Context(Object state)     at System.Threading.ExecutionContext.runTryCode(Object userData)     at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)     at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)     at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)     at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack)     at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state)  , Allocation call stack (if present) null

    Searched event viewer for related errors (inconclusive, no related logs found);
    Created a new blog site for testing a new approval workflow (e-mail not received);  
    Created an approval workflow in other site of the portal on which the approval user has access (e-mail not received);
    Created an workflow of other type (e-mail not received);
    Created an workflow of other type in other site of the portal on which the approval user has access (e-mail not received);

SharePoint version:
12.0.0.6524
WSS 3.0 SP2 12.0.0.6524
MOSS 2007 SP2 12.0.0.6524 with SP2