Capsule (2; 102; 202) containing a substance for the preparation of a potable beverage by extracting and/or dissolving the substance by means of supplying a fluid under pressure into the capsule (2; 102; 202), wherein the capsule (2; 102; 202) comprises an aluminum capsule body (12; 212) having a central capsule body axis, said aluminum capsule body (12; 212) being provided with a bottom, a side wall (16; 116) and an outwardly extending flange, the capsule (2; 102; 202) further comprising an aluminum cover attached to the outwardly extending flange, the cover hermetically closing the capsule (2; 102; 202), wherein the capsule (2; 102; 202) further comprises a sealing member at the outwardly extending flange for providing a fluid sealing contact with an enclosing member of a beverage preparation device if the capsule (2; 102; 202) is positioned in the enclosing member of the beverage preparation device and the enclosing member is closed by means of a closing member of the beverage preparation device, such as an extraction plate of the beverage preparation device, such that the outwardly extending flange of the capsule (2; 102; 202) and at least a portion of the sealing member of the capsule (2; 102; 202) are sealingly engaged between the enclosing member and the closing member of the beverage preparation device, wherein the enclosing member of the beverage preparation device comprises an annular element (41; 141) having a central annular element axis and a free contact end (30; 130), characterized in that, the sealing member is integral with the outwardly extending flange and comprises an annular projection (50; 150) comprising a projection top projecting axially towards the capsule body bottom from an inner projection foot radially outside of an inner flange portion and an outer projection foot radially inside of an outer flange portion, and wherein the outwardly extending flange comprises an annular trough between the inner projection foot and the side wall (16; 116), the annular trough having a bottom (52; 252) axially spaced from the outer projection foot towards the bottom of the capsule body (12; 212), wherein a radially outer wall of said projection (50; 150) is oriented at an enclosed angle relative to the capsule body axis of less than 8° and more preferably less than 6°, and wherein the outwardly extending flange extends from the free end of the side wall (16; 116) in a direction at least substantially transverse to the central capsule body axis, wherein the outwardly extending flange has no more than one single annular projection (50; 150), wherein an projection top end extends around the capsule body axis at a diameter of 31.9 to 32.4 mm, wherein the bottom (52; 252) of the trough is axially spaced from the cover, wherein the sealing member is deformable such that, in use, closure of the enclosing member causes the axial spacing between the outer projection foot and the bottom (52; 252) of the trough to be reduced, wherein the thickness of the aluminum capsule body (12; 212) is such that it is deformed easily if the capsule (2; 102; 202) is positioned in the enclosing member of the beverage preparation device and the enclosing member is closed by means of the closing member of the beverage preparation device, wherein the thickness of the aluminum capsule body (12; 212) is 20 to 200 micrometer, more preferably 100 micrometer, wherein the aluminum cover is arranged to tear open on the closing member of the beverage preparation device, such as an extraction plate of the beverage preparation device under the influence of fluid pressure in the capsule (2; 102; 202), wherein the outwardly extending flange comprises a curled outer edge, wherein the height of the sealing member portion to be contacted first by the free contact end (30; 130) of the enclosing member when the enclosing member is closed is at least 0.8 mm and at most 2 mm, and wherein the sealing member and the remainder of the capsule body (12; 212) are made of the same plate material.