Mustard should ideally be refrigerated, at least after opening. However, this does not imply that it will spoil if left out.
Since mustard is naturally antibacterial, it is shelf-stable. When you add vinegar, salt, etc., the expiration life is increased.
Once mustard seeds are exposed to air, water, and vinegar, they begin to release their 'heat,' which is irreversible and can only be slowed down.
Since mustard does not perish in the conventional sense, bottles and canisters typically bear a "best-by" date rather than an expiration date.
Even if the product has been stored in an unopened, sealed container, its flavor & potency will begin to deteriorate after this date, even if it has been properly stored.
Refrigerate your mustard after opening to maintain its flavor, and consume it within six months to appreciate it the way it was meant to taste.
Technically, mustard can last forever, but its essence will diminish over time. Use mustard by the best-by date on the bottle for optimal efficacy.
Though mustard does not go bad, crumbs of bread, swirls of mayonnaise, and other pollutants stuck in your mustard can create breeding grounds for germs.
To avoid contaminating mustard with other foods, always take it out with a clean spoon.