Great topic! Thanks for posting.
I would like to offer some first-hand experience here. I am responding to the OP and have not read subsequent posts, so if this has been covered, I apologize. I am short on time.
The first thing I would like to say is that I have been doing YouTube since 2007 and I have several active channels, and not once in all this time have I ever received an email offering sponsorship for any of my channels. So, I am very curious to know exactly what you mean when you say that you are being offered "Sponsorships"? My guess is that what you are talking about is not sponsorship, but you are being offered free products in exchange for a review. If this is what you are talking about, this is not sponsorship. Sponsorship is when a company offers to financially back your channel or a single video in exchange for you promoting them or their product. There are many YouTubers that have sponsors, and they generally will have a short cutaway in their video to talk about their sponsor. Getting a free product in exchange for you doing a video review of it, is not a real sponsorship of you channel, as no money is involved. So, if this is what you are talking about (not saying that it is), it is not sponsorship offers.
If the above is accurate and describes what you are talking about, I have years of first-hand experience in this and I can say from personal experience that 99% of these offers are not scams, and are legitimate. I have done literally hundreds of product reviews in response to these emails and not once have I ever been scammed. I am producing several videos of products I received for free right now, and there are more in the mail on their way to me. I am not saying that you do not have to be careful and there are not some scams out there, but these emails are generally from legitimate companies seeking legitimate reviews of their products. Most are Chinese companies and are using a translator program, so the fact that the English grammar is not great, is not a sign that it is a scam.
So let me give everyone some advice on how to handle these emails and how to do these types of reviews.
1. Never pay for the product in advance. Request the product to be mailed to you directly. If they will not do this. Do not deal with them.
2. In some cases they will offer you a 100% OFF discount coupon for amazon.com to buy the product. If you feel like doing this, go ahead, but never pay anything out of pocket.
3. Never promise a positive review in exchange for the product. Just tell them that you will produce an "Honest" review of the product. If they demand a 5-star review or positive review, do not deal with them.
4. Never promise to leave a review on Amazon.com for the product. This is against Amazon.com ToS and will get you and the seller banned from Amazon if you get caught, and you WILL get caught. Only agree to do an "Honest YouTube review". You are YouTuber, not an amazon.com reviewer.
5. If you do a product review video in exchange for e free product, you MUST!!!!! state this clearly and verbally in your video. This is required by the FTC. Simply clicking the "Paid Promotion" box is not enough, although you should click this as well.
6. If you provide affiliate links in the description of the video to products you MUST have a clear message BEFORE the links stating that the link is an affiliate link and that you may earn a commission if they use your links to buy products. The FTC has approved this shortcode, #AD, for use in places where space is limited and can not accommodate a full-length FTC Affiliate disclaimer message.
There is nothing wrong with doing honest product reviews in exchange for the product. Many small companies need exposure and a good honest YouTube review of their product can help them. If you get a product that is garbage, do an honest review of it. You are working for your viewers, not the company that gave you the free product. They are NOT paying you to produce a promotional ad for their product. They are giving you a product that in no way comes close to compensating you for the work and time you will put into reviewing it. You do not owe them anything just because they gave you a free product. They are the ones that owe you. What you are giving them is of massively greater value than whatever the cost is of the product they provided you with for the review.
If they want a promotional ad for their product, charge them professional rates to produce it. My rate to produce a promotion ad is $100 an hour plus expense with a $500 deposit to start. This is not a video I am producing for my YouTube channel. This is a video that they will own and they can do what they want with. I generally require them to provide me a script to produce the promo and using that script, I give them an estimate for the hours and expenses (Travel, actors, models, etc) to produce the promo. All edits and amendments to the original script are additional charges and hourly fees. Once the video is produced, they get a low-quality Watermarked sample to review. If they like the video, I request they pay me in full before a give them the full quality no watermaker video. If after they get the video, they want edits, I charge $100 an hour for the edits.
The reason I told you that last bit about producing a promo ad, is to help solidify in your mind that, receiving a free product in exchange for an honest review is not the same thing as making a promotional video. As a professional YouTuber, you are working for your viewers, not a company that gives you free products. No professional videographer on earth would produce a thousand-dollar video for a $30 free product. Okay? Once you qualify for the YouTube PP program, you ARE a professional videographer! Do not think so low of yourself as to give your talent and skill away for a cheap free product. The reason for producing the review is not for the free product, it is to inform your viewers about it. It is good fodder for your channel and your viewers will appreciate it if you are honest and do a fair review.
I hope this helps.