The thick mucus drooling from the nose on the back of your throat is the post nasal drip. Bacteria survive and grow rapidly when there is excess mucus production. In turn, they produce foul smell which is often terms as bad breath.
The blocked sinus results in post nasal drip which is a foul-smelling mucus drooling down from sinuses on throat through the nasal passage. Bacteria grow and thrive on post nasal drip and contributes to bad breath.
If you have noticed bad breath and irritation along with a whitish mark present at the back of your mouth, then there is a high chance that you have tonsil stones. A characteristic smell is noticed when you have tonsil stones.