1. A multi-part plastic closure with a lower part capable ( 1) of being put on a receptacle and comprising a pour-out mouth ( 18) extending upwards beyond a sealing region ( 16) with the receptacle, an annular wall ( 19) with an outer thread ( 20) on which a screw cap ( 2) is screwably held between a lower sealing position and an upper open position, the pour-out mouth ( 18) comprising a top surface ( 21) connecting to the annular wall ( 19) with a central pour-out opening ( 22), wherein a stiffener ( 23) formed in a rib-like manner forms a connection between the annular wall ( 19) and the top surface ( 21) to prevent a deformation of the pour-out mouth ( 18) under influence of an inner pressure, and the screw cap ( 2) having a central peg ( 29) which sealingly engages into the pour-out opening ( 22) in a closed condition of the closure, and at least one drinking opening ( 28) arranged between the cap lateral wall ( 25) of the screw cap ( 2) and the centric peg ( 29).

2. A multi-part plastic closure according to claim 1, wherein the screw cap ( 2) on an inner side of the peripheral wall comprises a retaining bead ( 30) at an end which cooperates with an annular abutment ( 31) which surrounds the annular wall ( 19) below the outer thread ( 20) and prevents a screwing-off of the screw cap.

3. A multi-part plastic closure according to claim 1, wherein the pour-out opening ( 22) has sealing means which cooperate with the peg of the screw cap ( 2).

4. A multi-part plastic closure according to claim 3, wherein the sealing means is a sealing lip which is integrally formed on a lower side of the top surface and which surrounds the pour-out opening at an edge.

5. A multi-part plastic closure according to claim 4, wherein the top surface ( 21) on a lower side comprises an annular groove ( 37) bordering the sealing lip ( 36).

6. A multi-part plastic closure according to claim 1, wherein the lower part ( 1) has a guarantee strip ( 35).

7. A multi-part plastic closure according to claim 1, wherein a protective cap ( 3) is pushable on the lower part ( 1) with one of a non-positive fit and a positive fit, past the screw cap ( 2).

8. A multipart plastic closure according to claim 7, wherein the protective cap ( 3) has a guarantee strip ( 35) and is held on the lower part ( 1) of the multi-part plastic closure.

9. A multi-part plastic closure according to claim 1, wherein a plurality of drinking openings ( 28) are arranged so that in the closed condition of the closure the drinking openings ( 28) lie at least approximately in a sealing manner on a remaining annular top surface ( 38) of the pour-out mouth ( 18).