No. "......the departure has been postponed."

" *has been postponed". Also, the use of "schedule" 2 times in the same sentence is a little strange. You could probably use "Departure" as suggested by 'amul' below.

It's grammatically correct yes, but do you mean that the schedule had NOT been posted. That would make more sense. If that's not what you meant, if the schedule was posted then you should not say but. 'But' is used to contrast what you've said...e.g I had work today but I didn't go. You can't use but if you're not going to contradict what you've said like you did in this sentence...e.g I had work to day but I went....that doesn't make sense. Sorry if this was just confusing