What type of envelope should you use for a serious business letter?

For a serious business letter, it's best to use a standard #10 envelope, which measures 4.125 x 9.5 inches. Opt for a high-quality, professional-looking envelope made from sturdy paper, ideally in a neutral color like white or cream. Make sure the envelope is clean and free of any logos or designs that could distract from the message. Additionally, consider using a business return address on the back for a polished presentation.