A very famous bodybuilder, who used to wear jumpsuits at expos, and who's very, very strong, once told me he had a contract with a giant supplement company all based on a handshake. For years, the checks always came in, he worked his butt off, and everyone was happy. Then all of a sudden, things started to change, maybe the company sold, and that contract they had was no more (at least that's what he thought). He could have sued, but didn't. I'm an attorney and there have been a great many times, I've salivated at the thought of suing a company (or two). A first year, no a first semester, law school student could unleash a lawsuit on a contract breacher that would make their head spin, but is it worth it? That's what it really amounts to. The cases I've contemplated would cost me more time and effort than they're worth, but what about my friend, whose entire family depended on that money? If the sponsorship is your livelihood, then the stakes are way higher.
Years ago, a person's word was everything. If a man agreed to terms, it was done. It was cast in stone. If a man told you he'd pay, he'd do whatever it took to live up to his promise. It wasn't just about doing the right thing, entirely, it was also about honor, his own personal honor. Those values have been severely eroded today, but there's still people who do business this way. I'm one of them. If you give me your word, that's good enough for me.
That being said, every email, every text message, and every voicemail that manifests the parties' mutual assent, of an offer and acceptance, provided there's adequate consideration, results in a fully binding and enforceable contract. You don't need a formal writing; sometimes, even a napkin with some scribbling on it, is all it takes.
With regards to sponsorships - it's on you. If you're talking about trivial amounts of money, then a handshake might suffice, but you're taking a chance. If it's your mortgage, car note, and/or the sole means for your financial survival, a contract might be the better choice.