The Postal Service issued an updated Publication 25: Designing Letter & Reply Mail in November 2018
This publication is intended to provide guidance for mailers who design
letter-size mailpieces, and especially automation-compatible designs.
Printers, graphic artists, forms designers, envelope manufacturers, and
computer programmers can find useful information from basic addressing to
technical specifications.
Click to download Publication 25
Pictured above is letter mail designed and printed to USPS specifications on the Neopost Mach 5 envelope printer. Click to learn more about the Mach 5.