Hi Michael & NotARubicon,
Personally speaking, I would always suggest using some type of formal written agreement (model release / assignment of rights / work for hire agreement ) but if you are not from the US you may have different areas of concern to be aware of.
Emailing with a potential host may confirm the intent of both parties but isn't really a substitute for a formal agreement on who owns the content, if/how the host was compensated or even the location if the guest host wants to take you to court for whatever reason.
A few other reasons to have a model release/assignment of rights -
How do you plan to deal with altering/re-editing the provided footage, doing promotions, licensing the footage to others or even a simple change of heart from the host?
What if the guest host wants to use the same content or just wants it removed from your channel?
What if the host decides to badmouth your channel and cause drama for you?
What if your channel earns 1 billion dollars and you have no documentation to prove you didn't promise 1 million to all of the hosts?
What if you are sued by a guest host, where will you go for court or mediation?
Emails will not save you from all things, but that little release can provide you with more protection than using nothing at all. Even if you agree to share the content you should still have a formal agreement on what the footage can be used for and where.
Should there be any legal action taken towards you a group of emails may not be enough to deal with hosts who turn into haters, last minute lawyers, travel, time lost and legal fees. A consultation and a customized "model release" with an entertainment lawyer who is familiar with your plans, youtube and copyright laws would always be a safe way to start.
Having no model release, performance agreement or assignment of rights is the very reason people get copyright strikes and/or lose their entire channel.
But if you cannot afford to consult with a lawyer I would at least suggest getting a customized model release and some agreement for trade or compensation. Even if the compensation is in the form of free promotion, like $50 bucks or a box of t-shirts, it's better than nothing and reminds the guest host of your level of professionalism towards the project.
Back in the day we used to get the model release/work for hire and make the actors confirm their identity and understanding of the agreement on the video directly.Then we'd cut it out and save that footage alongside the signed agreement.
My apologies for the long response & good luck 2 both of you!