I already have a subdomain mail. For the same Please share if you have a similar experience or wisdom. What's the best practice in your opinion?
The exception is if you're a bank, you might want to monitor those.
2) The .io TLD in the past was mismanaged, and has been pwned by security researchers. I don't consider it secure enough for SaaS use, and I won't signup for business services with companies that use it.
Besides that it really doesn't matter unless you're a million dollar brand.
There's better ways to spend $10 in your small business.
If you have a trademark on your brand name, and someone else tries to squat your name in another TLD, file a trademark dispute to force them to release it. (Assuming your trademark isn't a generic word like "apple" where someone could conceivably have something like apple.accounting without being confusingly similar)
One that looks nice for your website (ideally .com). One where your service actually runs. One to host user content (so if you insufficiently sanitize something, it can't be used to exfiltrate cookies or whatever from your userfacing domain).
I wouldn't get example-mail.com or example.bike unless those are actually relevant. Only get mispellings if your name is easily mispelled (but then, consider a different name), at least until you have a large number of users or dollars coming in. Each domain is individually inexpensive, but buying a lot isn't, and spending time on it isn't.
I think DDR0 makes a strong point, a determined malicious actor will always find a domain you haven't considered so defensively registering dozens of domains is usually only an action taken by large banks or significant brands. Unless you are likely to be impersonated or have a duty of care similar to that of a bank then a single/small number of domains is probably sufficient, especially for a startup. Buying additional domains can be done as you get larger and the threat of impersonation increases.
However, that doesn't mean you shouldn't monitor domain purchases that are similar to your own. Blacklisting domains that you believe have been purchased for nefarious goals can prevent your own employees from being duped in convincing phishing attacks and it is always good to occasionally remind customers/third parties of the domains you operate from.
Anyway, I could waffle about this for ages - there is more info on the above link and you can try your own domain out!
Like other "entrepreneurs", ;-), I also had the tendency to book domains when an idea comes to mind. If nothing else, I usually start writing about the topic and keep it there. I also tend to realize that I might not be pursuing it, and then abandon them.
Of course, the side-effect of this is also that I have sold quite a few domains. A quick calculation on the back of a napkin puts the income to about $25,000+. Two of the most notable ones I remember being a HTML5.(TLD), and a hackathon.(TLD). I remember giving them the grandfathered GSuite, and Twitter handles to the buyers.
Right now, I pretty much own all of the known TLDs for my family name. ;-)
If I received an email from mail-fb.io for example it wouldn't look legit to me. Any of their subdomain would be good.