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When it comes to sending off letters or documents, most of us settle for the traditional white envelope. But if you’re looking to expand your horizons, make sure you don’t expand them too far. According to the USPS, some envelopes should never be used for mail because they can potentially create complications. “Don’t use patterns, prominent flecks or shiny-coated paper for your envelopes,” the Postal Service warns. “Some types of paper interfere with the machines that read addresses.” To send your mail, the USPS has to know where to send it. Unfortunately, the technology the agency uses to read your post doesn’t make this easy. According to the Postal Service, automated mail-processing machines will be reading the addresses you include, and these machines “don’t leave much room for error,” the agency says. That’s a problem, because the accuracy with which these machines read and process the address listed does affect the speed and handling of your mail. “Double-checking that you’ve addressed your mailpieces correctly will help ensure they arrive where they’re supposed to,” the USPS says. RELATED: For more advice delivered straight to your inbox, sign up for our daily newsletter. Using the wrong type of envelope isn’t the only thing that could interfere with the automated mail-processing machines that are reading off the addresses on your letters. In order to make sure your mail is able to reach its destination, you should also avoid using reverse type such as white printing on a black background, letting parts of the address slip out of view, cutting off important information when using address labels, applying labels at a slant, and using punctuation in anything other than the hyphen in an extended ZIP Code. You should not include anything below the ZIP Code line either. “Automated mail-processing machines read addresses on mailpieces from the bottom up and will look first for a city, state and ZIP Code,” the Postal Service explains. Size also matters here. According to the USPS, envelopes must be rectangular and made of paper to qualify for letter prices, but they also have to maintain certain size requirements. A mailable envelope can be a maximum of 11 and a half inches long and six and one-eighths inches high. But you have to watch out for its thickness, too. Your envelope must stay flat, which means it can measure no more than one-fourths of an inch in thickness. If you fail to meet these standards, you could be subjected to a higher cost. “If your envelope can’t fit through USPS mail processing machines, or is rigid, lumpy or has clasps, string, or buttons, it’s ’nonmachinable’ and you’ll have to pay $0.30 more to send it,” the agency warns. “You’ll also have to pay more if your envelopes are square or vertical.” RELATED: USPS Will No Longer Let You Do This, Effective Immediately.