The joint must be airtight where a smoke pipe enters a flue, if it leaks, cement the porous joint. Clean-out doors on the smoke pipe elbows must make airtight seals when closed. If they're not airtight, flames will be visible and may come through openings. A smoke pipe must never enter the flue at a downward angle. It should be re-set so that it goes into the flue at an ascending angle of 90 degrees or more.
Horizontal pipes are the greatest collectors of creosote.
Inspect them frequently. Soot is flammable. Clear the soot yourself from any easily reached narrow space. A flue that doesn't draw the way it should can cause a smoky back-up in the stove and in your home. Check to make sure there are no obstructions.