Agreed, it's not something I would rely on. I was just intrigued to discover that there is some sort of undocumented queue for sent (as opposed to posted) messages.

Understood, thanks for your reply. I'm looking at some not-very-well-designed legacy code and trying to work out whether its attempts to deal with a potential deadlock are flawed. Given that it's over 20 years old and nobody has complained it's probably better to just leave it alone.