Connecting part for a plug-in connection system of a coupling device for automatically coupling a trailer vehicle to a towing vehicle, comprising: a contact support, a contact structure, which is attached to the contact support and can be brought into engagement with a contact structure of a complementary connecting part of the plug-in connection system in order to establish a plug-in connection with the complementary connecting part during automatic coupling, a closure flap, which is pivotable relative to the contact support, is preloaded with respect to the contact support and can be released from the contact support against this preloading with a pivoting movement when the plug-in connection is established, and a seal, which is arranged on the contact support and, for sealing off the contact structure from the exterior space, can be elastically deformed by a mating piece which presses onto the seal, wherein the mating piece is formed by the complementary connecting part of the plug-in connection system, the complementary connecting part pressing onto the seal, when the plug-in connection is established, wherein the contact support has at least one seal groove, which surrounds the contact structure, and the seal is formed from at least one sealing lip (132, 134), which is inserted into the seal groove and has a seal profile which is inclined towards a side facing away from the contact structure, so that the sealing lip (132, 134) is elastically deformed towards this side when the mating piece presses onto the sealing lip (132, 134), and wherein the sealing lip (132, 134) has a profile portion, which projects from the contact support and tapers as the distance from the contact support increases, characterized in that the mating piece is formed by the closure flap pressing onto the seal when the plug-in connection is released, and in that the seal groove defines a first groove space, which is given by a recess formed in the contact support, and a second groove space, which adjoins the first groove space and is widened in relation to the first groove space, wherein the sealing lip (132, 134) has a seal seat, which is mounted in the first groove space, and the profile portion of the sealing lip (132, 134), which profile portion adjoins the seal seat, is deformable in the widened second groove space. a contact support, a contact structure, which is attached to the contact support and can be brought into engagement with a contact structure of a complementary connecting part of the plug-in connection system in order to establish a plug-in connection with the complementary connecting part during automatic coupling, a closure flap, which is pivotable relative to the contact support, is preloaded with respect to the contact support and can be released from the contact support against this preloading with a pivoting movement when the plug-in connection is established, and a seal, which is arranged on the contact support and, for sealing off the contact structure from the exterior space, can be elastically deformed by a mating piece which presses onto the seal, wherein the mating piece is formed by the complementary connecting part of the plug-in connection system, the complementary connecting part pressing onto the seal, when the plug-in connection is established, wherein the contact support has at least one seal groove, which surrounds the contact structure, and the seal is formed from at least one sealing lip (132, 134), which is inserted into the seal groove and has a seal profile which is inclined towards a side facing away from the contact structure, so that the sealing lip (132, 134) is elastically deformed towards this side when the mating piece presses onto the sealing lip (132, 134), and wherein the sealing lip (132, 134) has a profile portion, which projects from the contact support and tapers as the distance from the contact support increases, characterized in that the mating piece is formed by the closure flap pressing onto the seal when the plug-in connection is released, and in that the seal groove defines a first groove space, which is given by a recess formed in the contact support, and a second groove space, which adjoins the first groove space and is widened in relation to the first groove space, wherein the sealing lip (132, 134) has a seal seat, which is mounted in the first groove space, and the profile portion of the sealing lip (132, 134), which profile portion adjoins the seal seat, is deformable in the widened second groove space.