The wine smells wrong. Before deciding the bottle is faulty, it helps to know which fault you are smelling, because one of the common ones usually passes with air and another is a reason to go back to the seller.
Cork taint: musty, moldy, muted
Cork taint comes mostly from a compound called TCA, which the Australian Wine Research Institute describes as having a distinct musty, moldy aroma. People often expect a strong smell. Mild cases are subtler than that: the wine simply goes quiet, the fruit disappears, and what is left tastes like damp cardboard or an old cellar.
Sensitivity varies between people by a large margin, so one person at the table can find a bottle undrinkable while another notices nothing. That disagreement is normal and is not evidence that the wine is fine. Cork taint is generally regarded as a flavor fault rather than a safety problem, and it will not improve with air or decanting.
Oxidation: bruised apple, sherry, flat
Oxidized wine has had too much contact with air, through a failed cork, long storage or an open bottle. The AWRI describes the progression as a dulling of aroma moving through cardboard and straw notes to sherry-like and maderized character. Color helps as a second opinion: whites deepen toward gold or brown, reds go brick and dull at the rim.
The overlap with cork taint is real, since both can read as cardboard in mild cases. If the wine smells of apple, nuts or sherry, oxidation is the better guess. If it smells of a damp basement and tastes of almost nothing, cork taint is.
Reduction: struck match and drains, often temporary
A reductive wine smells of struck matches, rubber or, in bad cases, drains. It is the opposite problem: too little air rather than too much. This one is worth patience. Pour a glass, swirl it hard, and leave the bottle for twenty minutes before deciding. Many reductive wines open up completely and taste as intended.
If it has not shifted after half an hour in a decanter, treat it as a fault like any other. Note that you tried, because a merchant will ask.
Heat damage: stewed fruit and a pushed cork
Wine that has been hot loses freshness and tastes stewed or jammy, sometimes with a flat, cooked edge. Two physical signs support the diagnosis: a cork pushed above the lip, and dried wine on the neck under the capsule. Low fill for the age of the bottle points the same way, which is why buyers read fill level as a condition signal, as Decanter explains.
Unlike the other three, this fault usually happened to your bottle rather than to the batch. Where the wine has been since it left the winery matters, which is why the storage record for your own shelves is worth keeping.
What to record, and when to go back to the seller
Record the fault at the table, not the next morning. The details that matter are small and easy to lose.
Producer, wine and vintage: Bought from and when: Stored where since: Opened on: What it smelled of: Did twenty minutes of air change it: yes / no: Color, if unusual: Verdict: cork taint / oxidized / reductive / heat / not faulty: Outcome: refund, replaced, poured away, kept:
Keep the bottle, the cork and what remains of the wine. Most merchants who accept returns want to smell it themselves, and an empty bottle proves nothing. Ask promptly and describe what you found rather than asking for a verdict.
Whatever the outcome, keep the note with the wine. If you own more bottles of it, the next opening is a test of whether the problem was that one cork or the whole case. The opening history guide covers keeping a separate note for each bottle rather than overwriting the last one.