The ends of the flaps don't seal. I think this makes the envelopes likely to tear open when they go through the machines at the post office and I worry about the security of the contents.
The product is nice to use as I mail out quite a few letters about our company to various clients. Pop the letter in and fold the flap and it's sealed.
I use these envelopes for all of my personal correspondence and for paying bills. I am very happy with the product and especially like the fact that I don't have to lick the envelope!