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Setting properties while composing an Outlook Mail without forcing TNEF, and referring to those properties after sending

I am having a similar problem to the one described in this previously asked question, but I am looking for a bit more detail in the answer as my lack of experience in this area doesn't allow me to fill in some of the blanks:

Tag Outlook MailItem with ID number before send without causing TNEF (RTF) send

Like the asker of that question, I would like to set a User Property, say it's named "XXXX", to a MailItem while the user is composing it, so that when the user later revisits that same MailItem in his or her Sent Items folder I could read the "XXXX" property's value again.

My additional questions would be as follows:

  1. Dmitry Streblechenko suggests using MailItem.PropertyAccessor.SetProperty, but wherever I see that used I see people using a schema link to represent the property. What would be the correct schema link to use if I want my property still to be known as "XXXX"? In other words, what would be the correct syntax to use in Dmitry's suggestion?

  2. I gather I could use a ".GetProperty" call to read the property later from the Sent Items, but the problem is that our add-in has been around for years, and users sent items are already full of MailItems with the original UserProperty "XXXX" in them. Will I still be able to use UserProperties to get property "XXXX" even if I use .PropertyAccessor.SetProperty to set them?

  3. Alternatively, if I kept using the original code to set UserProperties while composing the message, but I then used .PropertyAccessor.SetProperty("http://schemas.microsoft.com/mapi/id/{00062008-0000-0000-C000-000000000046}/8582000B", false) during the ItemSend, like the original poster is suggesting, then I understand the UserProperties are not sent out with the message, which is fine by me, but will they still be available when revisiting the message in the Sent Items folder? Eugene Astafiev suggests that it's safe to use that .SetProperty call, but he doesn't clarify whether it affects what happens to the message at the Sender's end.

My personal preference would be a solution that would a) allow me to send the item without having to force Outlook in any way to send the message in any particular format and b) be able to continue to access the properties in the users' inboxes as we always have through the MailItem's UserProperties, but I am aware that I may be asking for something impossible here.

Solution
    1. You need a property in the PS_INTERNET_HEADERS namespace. E.g. http://schemas.microsoft.com/mapi/string/{00020386-0000-0000-C000-000000000046}/x-my-prop.
    2. Yes if you specify the full DASL name.
    3. Yes, the user property will still be there in the Sent Items folder.