Smoking article mouthpiece for receiving an insert unit

A smoking article comprising:

• an aerosol-generating substrate and a mouthpiece secured to a downstream end of the aerosol-generating substrate, the mouthpiece comprising:
• a first hollow tubular segment of elastically deformable material;
• and
• a second hollow tubular segment adjacent to and upstream of the first hollow tubular segment, the second hollow tubular segment comprising a permeable peripheral portion of a fibrous filtration tow surrounding a hollow defining a chamber for receiving an insert unit;
• wherein the first hollow tubular segment defines an opening through which the insert unit can pass from the exterior of the mouthpiece into the chamber of the second hollow tubular segment, and
• wherein at least a portion of the first hollow tubular segment inwardly projects into the opening to define at least one elastically deformable projection.