Retrieve attachments from E-mail router messages never gives results on Post-Operation message

Hi there,

Have a strange issue and I hoped that someone can help.
We have a CRM Queue with an assigned email address in our CRM system. In this queue, emails with XML attachments are received and I'm now creating a plugin to process the XML attachments.

I created a Post-Operation plugin on the "Create" message for the Queueitem entity.  In my code I check if the new queue item is an email and then I use following code to look up all the attachments:

public EntityCollection GetAttachments(this IOrganizationService pService,  Guid pInput)
        {
            QueryExpression qxprAtt = new QueryExpression();
            qxprAtt.EntityName = "activitymimeattachment";
            qxprAtt.ColumnSet = new ColumnSet(true);

            ConditionExpression cxprAtt = new ConditionExpression();
            cxprAtt.AttributeName = "activityid";
            cxprAtt.Operator = ConditionOperator.Equal;
            cxprAtt.Values.Add(pInput);

            FilterExpression fxprAtt = new FilterExpression();
            fxprAtt.Conditions.Add(cxprAtt);

            qxprAtt.Criteria = fxprAtt;

            return pService.RetrieveMultiple(qxprAtt);

        }

When an email is received for that queue using the Exchange email router, the create queueitem plugin gets fired but the above code returns 0 attachments. If I open the mail in CRM, the XML attachment is there.

As a test I manually created a new email in CRM and added an XML as attachment.  Then I added this email to the CRM queue using the email form button "add to queue" and now the above code returns 1 and the email gets processed correctly.

Anyone knows what is happening here?