I can't open that .ext file.
You've noticed that it is just an example of the sybtax of the "command" to put a screenshot here in the forums, right?
Would that enable me to instruct the server to leave the messages forever?
Nope, they aren't related. I don't remember now how exactly you change it but try right click on mail panel and look for something like "show messaages from..."
Btw, the messages are disappearing only from the email client or from the server too?