A method of checking the height of a mail item relative to at least one dimensional threshold SH defining a change in postage. The mail items are advanced at a constant speed V along a reference surface. The presence of a mail item is detected relative to a given point of the reference surface. First and second times t.sub.1, t.sub.2 for which the same mail item is present at the first and second distances d.sub.1, d.sub.2 relative to the reference surface is detected. The ratio t.sub.2/t.sub.1 is compared with a value equal to (1-.epsilon.), where .epsilon. represents a correction coefficient of less than 1 that depends on the tolerances for detection of the presence of a mail item. The height of a mail item is considered greater than the dimensional threshold SH if, and only if, the relationship t.sub.2/t.sub.1>1-.epsilon. is true.