How long does a MailItem.Property[myProperty] live?

Question

I'm writing a TransportAgent for Exchange and want to tag a MailItem with some identifying information.

The MailItem class seems to have this functionality with the MailItem.Properties property; however, my question is to the length of time an entry would remain available.

e.g.

e.MailItem.Properties.Add(new KeyValuePair<string,object>("MyKey", "myValue"));

After this MailItem is delivered to the correct mailbox, if someone were to reply to the email, would the newly created reply retain that set value?

I think while writing this question out I have answered it for myself (which rationally would be no) but any practical experience and insight would be appreciated.