Airmail envelopes with red and blue stripes?

Question

are those envelopes with the red and blue stripes at the sides any different from normal plain white envelopes? i posted 3 letters with these envelopes, (those that have the red and blue slanted stripes at the edges) and none of it got posted. what's wrong??

Answer 1

They are for use with airmail and it could be that the post office refused to deliver them./