Try unrolling it and draping it over some rods to air out for a while.

send it back, maybe?

In my experience mold doesn't "air out." If the paper actually has mold in it, if you put it up anyway I fear that you'll get mold growing on or under the paper eventually.

If returning isn't a possibility then I would cut a foot or so off one of the rolls and aggressively air and dry it to see if that changes the odor. Do you see any signs of mold colonies? If not, it may be that it was just stored in a dampish, air-less place and has acquired that smell, but not actual fungal infection.

Wallpaper, even new stuff, has a stale/musty smell to me.

If a lot of air improves the odor noticeably, than that's what I'd do before hanging it.

One technique that is sometimes used with books that have acquired a stale smell is to close them up with a 3-5" deep layer (on the bottom of a plastic container) of non-clumping kitty litter (the clay stuff, unscented and poured in, first, not over the item). For wall paper rolls I'd use a tall container like a clean garbage can with a tight-fitting lid (or seal it up with tape.) Put the rolls on a rack above the litter and never in contact with the litter. Leave it sealed for a few weeks. (Air and dry them well first and use the litter to draw out the remaining smell.) Some recommend doing the same thing with solid air fresheners, but those smell so foul to me it's worse than stuffiness and I think you are just covering the smell up. The true source of mustiness is slight dampness which responds well to airing and dehydration.

I might even go so far as to loosely unroll the paper (so air can penetrate to all layers) and keep it in a dry area, or a room with a dehu going.

You might also do a test hang of your cut-off sample to see if re-wetting it with paste (wallpaper paste is famously ideal mold-food) brings the problem back. Leave it on a sample board in your intended room to see if anything awful grows on it.

Two caveats: both high heat (think ironing the paper) and freezing are sometimes recommended as cures for mold on paper. Aside from the risk of potential damage to the paper itself, both treatments have the possibility that they will create a rebound affect with resting spores of the mold, bringing it back worse than ever.

HTH,

L.

Old wallpaper is very likely to have hide glue as the adhesive.

It smells a little, even when brand new.

Thank you so much everyone, and especially Liriodendron for your comprehensive response. Rosie's Vintage offered to take the paper back, minus shipping, but she assured me that there was no mold, only mildew smell. I really love the pattern, which was hard to find, so I'm going to keep it and follow the steps that have been suggested. At $95 per roll, however, I think that the smell should have been disclosed prior to purchase, as I will now have to invest a fair amount of time eradicating the odor. She suggested that I use wheat paste and a non-yellowing matte sealer. Any comments on that?

"At $95 per roll, however, I think that the smell should have been disclosed prior to purchase"

If it is the smell of the paste it would be considered 'normal.'

I have never seen of heard of "vintage" paper that was pre pasted.

Wheat paste is a must, the matte sealer, why?
If you need to seal it for a bathroom or something ok. Here is a link where you can get these products. Bob is an expert at antique wall coverings, any questions just ask him What he sells as a dead flat varnish is an industry standard

Here is a link that might be useful: wall paper supplies

Thank you Christopher - I appreciate the referral.
When I asked about the smell, this is what Rosie's Vintage said:
"If it would give you peace of mind you could always use a non-yellowing matte sealer on the wallpaper before you hang it."

"I have never seen of heard of "vintage" paper that was pre pasted. "

It depends on exactly how old it is.

"Vintage" does not have a fixed time associated

If you varnish over the paper, make sure it's something you really like, because taking it down is likely to be a nightmare. That doesn't mean you shouldn't do it, just consider the consequences.

"If it would give you peace of mind you could always use a non-yellowing matte sealer on the wallpaper before you hang it."

BEFORE you hang it????? I would like to see that happen.

Vintage" does not have a fixed time associated

I suppose.