Sinusitis can be either acute (sudden) or chronic (long-term). With chronic sinusitis, the infection or inflammation does not completely go away for 8 weeks or more.
What causes sinusitis?

Sinusitis can be caused by three things:

• Viruses.
• Bacteria.
• Fungi.

When the lining of the sinus cavities gets inflamed from a viral infection like a cold, it swells. This is viral sinusitis. The swelling can block the normal drainage of fluid from the sinuses into the nose and throat. If the fluid cannot drain and builds up over time, bacteria or fungi (plural of fungus) may start to grow in it. These bacterial or fungal infections can cause more swelling and pain. They are more likely to last longer, get worse with time, and become chronic.

Postnasal Drip:
The excess mucus production that triggers postnasal drip has a number of possible causes, including:

• Colds
• Flu
• Allergies (called allergic postnasal drip)
• Sinus infection or sinusitis (inflammation of the sinuses)
• Object stuck in the nose (more common in children)
• Pregnancy
• Certain medications (including some birth control pills and blood pressure medications)
• Deviated septum (abnormal placement of the wall that separates the two nostrils) or another anatomical problem that affects the sinuses
• Changing weather fronts, cold temperatures, or excess dryness in the air
• Certain foods (for example, spicy foods may trigger mucus flow)
• Fumes from chemicals, perfumes, cleaning products, smoke, or other irritants

Source: Postnasal Drip: Causes, Treatments, Symptoms, and More
See also 3-Step Silent Reflux Prevention Plan | The Dr. Oz Show

Can a Sinus Infection Be Caused by a Tooth? | Oral Answers says,
Hupp's Contemporary Oral and Maxillofacial Surgery textbook states, "Periapical or periodontal infections of maxillary posterior teeth may erode superiorly through the floor of the maxillary sinus. Approximately 20% of cases of maxillary sinusitis are odontogenic."

Basically, that's a fancy way of saying that tooth and gum abscesses of the upper back teeth can eat through the bone and invade the maxillary sinus. It further says that about 20% of all maxillary sinus infections are caused by tooth infections, rather than another cause.

Awful smell / taste after root canal:
I had an awful smell coming from what I thought was a drain in my gums from an infected tooth, but after a root canal and that drain healed up I was still getting the taste.

Some posts said,
"Mucus is a protein, and these bacteria feed on the amino acids cysteine and methionine that are found in proteins. As they feed, anaerobic bacteria produce metabolites that have characteristically noxious smells. One set of smells is related to sulfur compounds. This is the source of that characteristically "rotten egg" smell associated with bad breath secondary to post-nasal drip. Another set of smells is associated with polyamines that produce the classic putrescent odor of decay." Source: TheraBreath.com

Decongestants often have a rebound effect, particularly decongestants that contain benzalkonium. Such decongestants may prove effective initially. However, when the symptoms return within a day or two they will be worse than they originally were.

See The Sinus Flush - MedHelp
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground, or you can kneel down and lean over a bathtub, or perhaps a plastic basin or tub, with the shower running for steam

If you don't wish to take the risk involved, then at least you may wish to try the Dr.Grossan Hydropulse Nasal Irrigation System. It gently pumps the saline solution into your sinuses, and I don't know if it pumps saline all the way up into the far reaches of your sinuses, because of gravity, but it follows the same line of treatment expressed here. Treatment of Sinusitis:
Viral sinus infections usually go away on their own within 10 to 14 days. Antibiotics don't work for viral infections. But there are some things you can do at home to help relieve your symptoms:

• Drink plenty of fluids.
• Try over-the-counter pain relievers and decongestant pills to help relieve the pain and pressure in your head and face.
• Put a hot, damp towel or gel pack on your face for 5 to 10 minutes at a time, several times a day.
• Breathe warm, moist air from a steamy shower, a hot bath, or a sink filled with hot water.
• Use saltwater nose drops to help keep the nasal passages open and wash out mucus and bacteria.

Bacterial infections can be treated with antibiotics. You will probably feel better in a few days, but some symptoms may last for several weeks. You may need to take the medicine for a longer time if you have chronic sinusitis.

If you have a fungal infection--which is not common--antibiotics won't clear up your sinusitis. With this type of infection, you may need treatment with antifungal medicines, steroid medicines, or surgery.