title: Custom Properties against an Outlook Interop MailItem
body: <p>I am adding custom properties to a Microsoft.Office.Interop.Outlook.MailItem object like so:</p>
  
  <pre><code>Public Const SharePointSiteUrl As String = "http://schemas.microsoft.com/mapi/string/{00020329-0000-0000-C000-000000000046}/SharePointSiteUrl"
  Public Const SharePointSiteFolder As String = "http://schemas.microsoft.com/mapi/string/{00020329-0000-0000-C000-000000000046}/SharePointSiteFolder"
  Public Const SharePointSiteUsername As String = "http://schemas.microsoft.com/mapi/string/{00020329-0000-0000-C000-000000000046}/SharePointSiteUsername"
  Public Const SharePointSitePassword As String = "http://schemas.microsoft.com/mapi/string/{00020329-0000-0000-C000-000000000046}/SharePointSitePassword"
  </code></pre>
  
  <p>...</p>
  
  <pre><code>email.PropertyAccessor.SetProperty(OutlookHelper.SharePointSiteUrl, sharepointSite)
  email.PropertyAccessor.SetProperty(OutlookHelper.SharePointSiteFolder, sharepointFolder)
  email.PropertyAccessor.SetProperty(OutlookHelper.SharePointSiteUsername, sharepointUserName)
  email.PropertyAccessor.SetProperty(OutlookHelper.SharePointSitePassword, sharepointPassword)
  </code></pre>
  
  <p>After the four calls above have been executed, I test if I can access the properies using PropertyAccessor.GetProperty, and they are there.</p>
  
  <p>I then save the email to a directory using .SaveAs. When I open the mail item later and try and access the properties I get the following error:</p>
  
  <pre><code>email.PropertyAccessor.GetProperty(SharePointSiteFolder)
  
  The property "http://schemas.microsoft.com/mapi/string/{00020329-0000-0000-C000-000000000046}/SharePointSiteFolder" is unknown or cannot be found.
  </code></pre>