John Walter "Duster" Mails (October 1, 1894 – July 5, 1974) was an American pitcher in Major League Baseball.

Mails started his professional baseball career with the Seattle Giants of the Northwestern League in 1914.

The Robins won the pennant, but Mails did not appear in the 1916 World Series, which they lost.

[2][3] In 1917, Mails joined the Portland Beavers of the Pacific Coast League, pitched 49 innings, and went 3–2.

[3] He served in the U.S. military during World War I and did not play professional baseball in 1918.

[1] In 1919, Mails split time between the PCL's Seattle Rainiers and Sacramento Senators.

[3] In August, Sacramento traded Mails to the American League's Cleveland Indians.

In Game 3 of the 1920 World Series, Mails pitched 6.2 innings of relief, allowing no runs.

Mails spent the rest of the season with the International League's Syracuse Stars and the PCL's San Francisco Seals.

[3] In the major leagues, Mails had 516 innings pitched, a 32–25 record, a 4.10 ERA, a 101 ERA+, and 232 strikeouts.

He was nicknamed "Duster" early in his career with Brooklyn because of his wildness while on the mound.

During the last day of the PCL season in September 1935, Mails called his outfielders in before striking out the final batter of the game.