Some sellers make offers to watchers. Their offers are also in your eBay mailbox, so you can find them there. Ebay does not reject such offers, but if someone else accepts the offer or buys the item (BIN) the offers won't be available any more.
Or do you mean that you are a seller and someone made you an offer that was automatically rejected? Some sellers may automatically reject offers under a certain price or percentage of the BIN price, so it must be in your settings. I have never heard of eBay automatically rejecting offers, but I don't know for sure. If it does not tell you who made the offer that was automatically rejected, you might not be able to contact them and can only hope they increase the offer.
