Why do you keep getting your mail returned? Everything seems in order as both the return and mailing address are correct. Well, you would be surprised that there could be an error of how the text is placed.
With that guide, you can make sure that you keep within US Postal Regulations. There's a lot to consider.
But one thing that isn't mentioned in that blog is the return address should stay within an 1.75" from the top and then keep the mailing address below this line. This way there is no interference when the machine scans the addresses.
Remember, it's important that you get the envelopes right because an error can cause a lot of troubles. When your client starts mailing out your envelopes and then he gets them returned because the US Postal specs are wrong, you'll be in a world of hurt.