Rettas are a group of postmarks distinguished by a specific inscription, including a determined number of dots or inscriptions and used at the beginning of 1866. They were then replaced with ordinary postmarks. The classic retta, Type B-1, is a diamond composed of 81 diamond-shaped dots with points. Rettas later differed in details of size, shape, and number and form of the dots. Their initial use was to cancel stamps, the date-stamp being struck on the envelope. Beginning in about April 1866 this use of the retta was abandoned and date-stamps were used as obliterators. The rettas thereafter had the function of cancelling stamps that for any reason had not been cancelled at the point of origin. The circumstances included mail received from ships, stamps overlooked at origin, mail from civilian internment camps in war time, and even stamps that were already cancelled, but too faintly.

The retta may be found cancelling non-Egyptian stamps when they were used on board a ship arriving at an Egyptian port. After the classic period many variations on the retta came into use.